This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
Groundwater is an important natural resource for agricultural, industrial, drinking and domestic purposes. Over 35 of public water supply in England and in Wales and 70% of water supply in south and east England comes from groundwater resources (Stuart and Smedley, 2009). It has long been seen as a relatively pure natural resource that is stored in subsurface aquifers and its quality under threat from anthropogenic influences. Chemical quality changes occur mainly through direct input of anthropogenic substances during groundwater abstraction, resultant changes in the groundwater flow regime and in processes of artificial recharge (Stuart and Smedley, 2009).
Carbonate rocks form important subsurface aquifers in many areas of the world, especially north-west Europe where the Chalk is a primary source of potable water (Bloomfield, 1997). Chalks are major aquifers widely exploited in the United Kingdom for public water supply. Chalk groundwater plays an important role in the maintenance of flow both in national and international hydrological systems (Ander et al., 2004). The majority of public supply Chalk boreholes are situated along river valleys usually to exploit the shallow depth to the water table and higher transmissivity in the river valley (Stuart and Smedley, 2009). The Chalk is a fractured rock having very fine-grained matrix. The nature of its matrix makes much of the water held within the chalk pores not able to be drained by gravity. Aquifer properties of the Chalk are controlled by fractures and larger pores (Bloomfield, 1997). This gives rise to a highly transmissive, but relatively low - storage aquifer that is at constant risk from contamination by sea - water or agrochemicals (Jones and Robins, 1999). Alternatively, as the fracture system becomes less hydraulically significant, the matrix porosity is envisaged to increasingly influence solute movement (Bloomfield, 1997).
"Hydrogeochemical evolution of groundwater from its composition as rainfall, is dependent on the complex interplay of the processes taking place in the unsaturated zone and overlying composition and the composition of the aquifer and existing groundwater" (Ander et al., 2004). Therefore, knowledge of the chemical composition of pore water reflects both the origin of the groundwater and the way in which the original composition has been modified by diagenetic reactions with either rock or organic matter (Hancock and Skinner, 2000). Pore water fills the interstices of sediments and sedimentary rocks. Chloride, sodium and calcium are the most common ions found in pore water of deeply buried carbonate sediments (such as Chalk), while other elements are of lesser amount. Fluid mixing and reactions with carbonate minerals and clays exert an important control on the composition of pore water in Chalk aquifers. Intrusion of saline waters into pore water also influences the contribution of groundwater movement in the matrix of the Chalk. Due to the fine-grained nature of the Chalk matrix, small size of its pore throats and high pore water suctions, the pores cannot fully drain. However, since pore water pressures are less than atmospheric pressure, the chalk above the water table is described as unsaturated (Ander et al., 2004).
The study area, Morestead Road Waste Water Treatment Works (WWTW), is located on the Chalk downland of Hampshire, east of Winchester, and is split into two parts by the M3 motorway (Mortimore, 2004). Pore water samples extracted from core from boreholes within the study area will be investigated analytically for groundwater quality changes in the Chalk aquifer profile. Also, this study seeks to establish the relationship between water level changes and effluent discharge, to see whether consistency exists in the time taken to reach the water table as it's known about Chalk aquifers.
Morestead is a small village located near the South Downs in the city of Winchester district of Hampshire. It lies 5 km southeast from its county town, Winchester. The site's oldest part is situated west of the M3 motorway, north easterly slope of St. Catherine's Hill (SU 486 277) and boreholes from here are designated by MWC. This location sits on the steep slope of the hill leading northwards down to the River Itchen via a small stream that drains westward out of Morestead Road WWTW ponds. The site's other part is situated east of the M3 motorway towards the northern flanks of Twyford Down (SU 492 277) with boreholes extending from here labelled MWE. The Morestead WWTW is positioned southwards off an east - west valley formed by an erosion along the crest of the Winchester Anticline. High ground surrounding this local valley rises to and above 140mAOD.
Brydones map 2
Figure 1: Geologic map of Chalk zones within and around Winchester, Hampshire. It shows the domal structure of the Winchester Anticline with an east - west axis plunging both east and west, forming a pericline. The corresponding Chalk Formations are clearly indicated by the arrow signs, while the red dots are: the Morestead WWTW sites and St. Catherine's Hill WTW (adapted from Mortimore, 2004).
1.3 GEOLOGY OF THE AREA
Winchester area provides a continuous exposure through the middle and late chalk of the Cretaceous (Turonian to Campanian stages) along the east, through the M3 motorway. Exposure of the upper Turonian stage can be seen near the Bar End, while the Coniacian and Santonian stages exposed through Twyford Down and partly near Shawford. Adjacent to the area of Morestead Road WWTW, cuttings of the M3 motorway show the strata dip and general geological structure in the White Chalk Subgroup, particularly the Lewes Nodular Chalk, and the Seaford Chalf Formations (Mortimore, 2004). A northerly dip of 150 was identified on the northern limb of Winchester Anticline, measured on the navigation Marls in the Lewes Nodular Chalk exposed under the Spitfire Chalk Formation (dipping north). The topmost New Pit Chalk of the motorway cuttings of the Twyford Down, expose the entire Lewes Nodular Chalk and basal beds of the Seaford Chalk Formation. The Southerham Marl dip measured in Twyford Down is 60 - 80 southwest on the southern limb of the Winchester Anticline (Mortimore, 2004).
Despite good motorway sections in the Chalk, few exposures of the lower formations forming the bedrock chalk in the area are around New Morstead Road WWTW. Previous investigations of five shallow depth (10 to 15m) rotary - cored boreholes (October, 1996, logged by Professor Rory Mortimore) at Morestead Road WWTW indicated, in downwards succession, the presence of basal Holywell Nodular Chalk including the Plenus Marls and Melbourn Rock, and the Zig Zag Chalk Formation, known as the White Bed (Mortimore, 2004).
Brydone (1912) zonal map of the Chalk of Hampshire was based on fossil evidence and show the site to situated on Holaster Subglobosus Zone (now broadly referred to as Zig Zag Chalk Formation) and the Inoceramus Labiatus Zone (known as Holywell Nodular Chalk Formation). He also numbered the exposures of chalk where he obtained fossil evidence for the zones measured above. Three of such numbered localities (57, 79, and 80 and 117) are still of relevant to the Morestead Road investigations.
C:\Users\Christopher\Desktop\Liman Data\Data for Liman\Morestead Rd Maps\Morestead_new_geol_crop.JPG
Figure 1.2: Generalised geology of Morestead Road WWTW site, showing the different borehole locations (adapted from British Geological Society (BGS) Map 299 Winchester, 2001).
1.4 REGIONAL STRATIGRAPHY AND HYDROGEOLOGY
Originally, the UK chalk was divided into three, namely: the Upper, Middle and Lower Chalk. The nature of this broad distribution incorporated significant thicknesses of Chalk, for which recent mapping has provided subdivision of these Formations into mappable units of member status (Bristow et al., 1998). These members are primarily defined into marker horizons such as marl layer, hard grounds and flint layers. A combination of the marker horizons, colour, hardness, geomorphology and fossil content define distinctive characteristics of each unit. Table 1.1 below show the relationship existing between the new and old stratigraphical classifications with respect to the Southern Province.
Table1.1: Tabular relationship between old and new Chalk stratigraphy for the southern geological province adapted from Bristow et al. (1997)
The Hampshire Chalk water table generally follow a subdue form of topography with the unsaturated zone thicker under hills and thinner in valley regions (Allen at al., 2009). Chalk confined by Palaeogene strata have the Chalk piezometric surface not shown and limited data from boreholes indicate a gradient towards the coast. In Chalk aquifers, flow takes place either via fracture flow of matrix flow or a combination of both. Vertical and horizontal flow components may be responsible for aquifer recharge through the unsaturated zone. Recharge processes are likely to be dominated by the presence of permeable and impermeable horizons within the Chalk (Stuart and Smedley, 2009). The occurrence of impermeable layers also helps to concentrate flow in certain areas.
Soil physics analysis from a farm near Winchester (Bridget's Farm), concluded that water movement predominantly occur through fine pores of the Chalk matrix with only a minor component occurring through fissures (Wellings, 1984).
Lithology is an important control on the transmissivity of the Chalk and the hydraulic properties of Chalk are greatly influenced by the presence of hard grounds, flints and marl bands. Fissure zones in the Chalk generally appear to be related to tabular flint bands in the Upper Chalk (Entec, 2002). Some karst features have been observed in the Twyford cuttings on the M3 motorway (Hopson, 2000).
The Upper Chalk is general considered a better aquifer than both the Middle and Lower Chalks. However, solution can increase the hydraulic properties of the Middle Chalk where it is exposed near the surface. The high marl content and limited fracture development of the lower Chalk makes it a poorer aquifer (Giles and Lowings, 1990).
Topography also is a strong influencing factor on the hydrogeological system of Chalk aquifers. Yields from boreholes in interfluves areas are generally less than those in valleys (Allen et al., 2009).
Geological structures, such as faults and folds, also play both direct and indirect role in aquifer property development. Giles and Lowings (1990) suggested higher yields along the axes of weathered synclines rather than in anticlines.
The porosity of chalk varies between 5% and 45% and is dependent on the stratigraphy (Bloomfield et al., 1995). The Upper Chalk of Southern England has an average porosity 39%, while the Middle Chalk and Lower Chalk are 28% and 23% respectively (Bloomfield et al., 1995). At Twyford, the Upper Chalk porosity of the Seaford Chalk is in the range 38 - 50% and that of the underlying Lewes Nodular Chalk is between 35 and 40% (Stuart et al., 2008a).
A study of artesian boreholes at Alresford (in the Candover catchment) by Headworth (1978) showed that a narrow zone at the top of the boreholes contributed the majority of flow, with the remaining aquifer providing upward leakage to a high transmissivity layer. At Twyford, packer test of a borehole found that the most important flow horizon occurred in the zone extending 10m below the water table, with decreasing values below this (Stuart et al., 2008a).
Transmissivity values are lower than 500m2 day-1 across the groundwater divide approaching the Weald in east Hampshire, reflecting the stratigraphy of the Chalk. Groundwater flow predominantly occurs through the Lower Chalk (less fractured) and storage potential reduced as well. On the scarp slope towards the Thames Basin, in the north, transmissivity improves (Allen et al., 2009).
1.5 INTRODUCTION TO THIS DISSERTATION
CHAPTER TWO: LITERATURE REVIEW
2.1 PROPERTIES OF CHALK AQUIFERS
Chalk is a sedimentary rock with diagenetically altered calcareous nannofossils constituting its main component. It is usually derived from calcareous ooze of the ocean where nutrient and temperature conditions of the surface waters favour calcareous plankton (Berger and Winterer, 1974). The depth of formation of halk deposits is controlled predominantly by the solubility of dead remains of calcitic plankton in the ocean water, described as the Carbonate Compensation Depth (CCD).
The presence and development of fractures in the Chalk, gives it the properties of an aquifer. The permeability and specific yield of the Chalk would be negligible without fractures. Furthermore, without the presence of solution enhancement of the fractures, the high transmissivity of the Chalk would not have been possible, and without further groundwater flow concentration and dissolution of chalk, karstic and conduits features would not be observed. The Chalk often is referred to as possessing "dual porosity" (Price, 1987, Barker, 1991, Price et al., 1993). The matrix pores provides storage in a classic dual porosity aquifer while the fractures provide the permeable pathways to permit groundwater flow. Movement of groundwater within the Chalk is more complex, i.e. high porosity (produced by coccoliths) is not readily drained, due to the very small pore throats (Price et al., 1976). Therefore effective groundwater storage primarily occurs within the fracture network and larger pores.
Woodland (1946) studying the hydrogeology of East Anglia identified the relation between transmissivity and topography. Ineson (1962) then estimated transmissivity from specific capacity data and produced maps of transmissivity variation over the Chalk. The most distinct feature of these was the correlation or agreement of high transmissivity values with valley bottoms, particularly at valley confluences. More recently, renewed interest in the regional distribution of aquifer properties have arisen, due to the development of numerical models (e.g. Rushton et al., 1989, Cross et al., 1995). The majority of these models require information on aquifer properties from the interfluves as well as from the valley bottoms.
Many factors have contributed to the development of aquifer properties within the Chalk. General topographic pattern of transmissivity variation through a number of processes have developed, such as: groundwater flux concentration within valleys (Rhoades and Sinacori, 1941, Robinson, 1976, Owen and Robinson, 1978, Price 1987, Price et al., 1993); Chalk structure (Ineson, 1962, Water Resources Board, 1972, Price, 1987, Price et al., 1993); and periglacial erosion, particularly within taliks (Higgenbottom and Fookes, 1971, Williams, 1980, Gibbard, 1985, Williams, 1987, Younger, 1989).
As described earlier (see section 1.4), the Chalk lithology has an important effect on aquifer properties, especially the presence of marl layers, flints or hardgrounds (Price, 1987, Buckley et al., 1989, Lowe, 1992, Mortimore, 1993, Bloomfield, 1997). As can be seen in Figure 1.3, lithology and structure geomorphology, such as valleys and dissolution enhanced fractures, promotes transmissivity in Chalk aquifers. Palaeogene, or other younger cover, can be influential in the development of solution features and groundwater conduits, as can recent or historic presence of rivers, and periglacial activity (Fagg, 1958, Atkinson and Smith, 1974, Walsh and Ockenden 1982, Price et al., 1992, Banks et al., 1995, MacDonald et al., 1998, Lamont - Black and Mortimore, 1999).
Figure 2.1: Some factors that contribute to the development of aquifer properties within the Chalk (adapted from MacDonald, 2001).
2.1.1 CHALK MATRIX
The frequent and well connected nature of the pores within Chalk matrix results in enhanced porosity but the particular small pore-throats causes very low permeability (Price et al., 1993). Average permeability of 977 measured core samples from the English Chalk was found to be 6.3 x 10-4 m d-1 (Allen et al., 1997).
Bloomfield (1997) identified two main diagenetic processes to be responsible for Chalk porosity even though porosity in Chalk is thought to be variable and affected by lithology but primarily determined by the degree and nature of diagenesis. The two main diagenetic processes are; mechanical compaction (physical rearrangement of fragments to form denser structures) and pressure solution or chemical compaction (dissolution and re-precipitation of minerals). Following the initial accumulation of biogenic sediments, porosity is believed to be between 70 and 80% (Hancock, 1993). The effect of bioturbation reduces porosity to about 60% (Bloomfield et al., 1995), while later diagenetic processes and mechanical comapaction, after burial of about 250m, reduces the porosity to about 35 to 50% (Hancock, 1993). As the overburden increases, chemical compaction becomes more important than mechanical compaction and burial of about 1000 m reduces porosity to approximately 30 - 40 % (Bloomfield, 1997, Hancock, 1993).
Chemical compaction processes have a more variable effect on porosity than mechanical compaction and depend upon pore water chemistry, lithology and clay mineral content (Bloomfield, 1997). Allen et al. (1997) evaluated porosities of four areas of the English Chalk (Table 2.1)
Table 2.1: Porosity estimates in four areas of the English Chalk (as adapted from Allen et al. 1997)
Northern England Upper Chalk
Norther England Middle Chalk
Southern England Middle Chalk
Southern England Lower Chalk
Thames and Chilterns Lower Chalk
Thames and Chilterns Lower Chalk
Southern England and Thames and Chilterns Upper Chalk
East Anglia Upper Chalk
East Anglia Middle Chalk
2.2 REGIONAL DISTRIBUTION OF AQUIFER PROPERTIES
The four regions of interest are: (1) Southern; (2) the Thames Basin (including the North Downs); (3) East Anglia; and (4) Yorkshire and Lincolnshire. MacDonald (2001) showed that the transmissivity distribution, over the four regions above, is broadly similar.
Giles and Lowings (1990) suggested a connection between folding in the Hampshire area and high transmissivity. However, lower transmissivity areas of Dorset and the South Downs have been subjected to more folding than either Hampshire or Salisbury Plain. The lowest median transmissivity values are found in East Norfolk and East Suffolk, London areas. These three areas of the Chalk aquifer have considerable cover and are confined in some places.
2.2.1 CHALK PROPERTIES IN VALLEYS AND INTERFLUVES
Generally, it is accepted that borehole yields in the Chalk are higher in valleys than over interfluves and that transmissivities tend to follow in similar pattern (e.g. Woodland, 1946, Ineson, 1962). One easy ways to test transmissivity variation in valleys compared to interfluve is to examine the correlation between transmissivity and depth to rest - water - level (RWL). This is because RWLs are known to be shallower in valleys than interfluves.
Robinson (1976) predicted transmissivity variations in the area under low water table conditions using factors such as primary distance from winter flowing streams modified by depth to minimum RWL and the Upper and Middle Chalk thickness. A good correlation between the predicted result and those from high quality pumping test, carried out under minimum water table conditions, was achieved.
2.2.2 CHALK IN CONFINED AND UNCONFINED CONDITIONS
MacDonald (2001) using database to examine variations in both transmissivity and storage coefficient, with the degree of confinement of the aquifer, observed that the storage coefficient exhibited significant differences between measurements taken in confined and unconfined conditions. He found that the median for confined pumping tests was 0.0006 and 0.008 for unconfined tests. Semi - confined tests had values in-between. Values from the confined Chalk agreed favourably with detailed research into Chalk storage undertaken by Lewis et al. (1993). They calculated the specific storage of the Chalk to be 1.5 x 10-5 m-1, which when integrated over a 100m thickness of Chalk aquifer, gives a storage coefficient of 0.0015 for confined Chalk.
Storage coefficient estimated from unconfined pumping tests show similarity to those estimated from groundwater - level recession analysis by Lewis et al. (1993). For which the top 30 m of saturated Chalk, was estimated for storage coefficient to be 0.005 to 0.05, comparing favourably with the unconfined pumping test inter-quartile range of 0.0028 to 0.017.
Identified difference between confined and unconfined storage elucidates the relative importance of elastic storage in the Chalk. Chalk aquifer are highly compressible (Carter and Mallard, 1974, Price et al., 1993) explaining the high estimates of specific storage by Lewis et al. (1993) and the corresponding high values of storage coefficient evaluated from confined pumping tests. Specific yield is limited by small proportion of Chalk pores in unconfined aquifers that can drain under gravity - less than 1% of the Chalk bulk volume (Price et al., 1976, Price, 1987), for which the limited volume of groundwater is stored in fractures. Therefore, under unconfined conditions, the elastic storage is still relatively important.
Understanding th variation in aquifer properties is fundamental to modelling groundwater occurrence and contamination movement pathways within the Chalk aquifer (MacDonald, 2001).
2.3 UNSATURATED ZONE FLOW PROCESSES IN CHALK
Much debate has occurred on how much flow in unsaturated zone in the Chalk is through fissures and fractures compared to matrix. Smith et al. (1970) suggested about 85% of flow in unsaturated zone occur predominantly via the matrix for which he introduced the concept of piston flow (water within the unsaturated zone is gradually displaced by new recharge given rise to the oldest and deepest groundwaters to be discharged through the bottom of the aquifer system), accounting for the rapid response of rainfall regardless of the slow migration of tritium. An average of 1 m/year was calculated for the downward movement rate through unsaturated zone by Smith et al. (1970).
More recent tracer studies such as those carried out by Welling (1984), Barraclough et al. (1994), Haria et al. (2003) and Van den Daele et al. (2007) have shown a peak concentration move through zones of unsaturation at about 1 m/year. Also, near surface measurement of soil moisture content and matric potential (e.g. Wellings and Cooper, 1983, Ireson et al., 2006) have been used to investigate recharge mechanisms for which a conclusion of flow through the matrix was determined as the dominant process, with fissure flow initiated at some sites where the matric potential is beyond some certain threshold.
Wellings and Cooper (1983) confirmed three of these sites studied at Hampshire show matrix flow as the dominant process while fissure were initiated during wet periods. However, this view was questioned by Mahmood-ul-Hassan (2002) on the basis that the frequency of data collection affected the results Welling and Cooper (1983) obtained. They demonstrated that using higher frequency data, rapid changes in water content and matric potential, in a matter of hours following rainfall, indicated preferential flow through fissures and fractures as against a low frequency data that indicated matrix flow only at the same time.
Barraclough et al. (1994) carrying out a tracer investigation of the top 6 m of the unsaturated zone in Berkshire did not identify fissure flow. However, the absence of tritium peak at 15 - 20 m depth suggested that vertical flow may be more dominant going deeper into the unsaturated zone.
Price et al. (2000) measuring Chalk block properties, suggested that fissure flow can be generated at any depth within the unsaturated zone, particularly in areas where matrix hydraulic conductivity is low. Unsaturated zone fissure flow was thought to be initiated in narrow parts of fissures and where more likely to originate near the water table with the lowest pore suctions.
2.4 HYDROGEOCHEMISTRY OF THE CHALK AQUIFER
Groundwater chemical compositions are controlled by a number of factors, the principal of which is carbonate reaction. It occurs rapidly leading to a strong buffering on the groundwater composition. Geological structures and weathering patterns are influential in controlling groundwater flow and flow rates resulting in chemical variations, both with depth and laterally, particularly in the confined near - coastal parts of the Chalk aquifer.
Unconfined aquifers generally have groundwater emanating from them to be aerobic with observed concentrations of dissolved oxygen up to saturated values of around 10 mg l-1 and redox potentials (Eh) often above 300 mV (Adams, 2008). Groundwaters from unconfined Hampshire Chalk have little variation in major-ion composition and are mainly of Ca-HCO3 type (typical of Chalk groundwater).
2.4.1 MAJOR CONSTITUENTS
Concentration of Ca and HCO3 in the Chalk aquifers are of an order of magnitude higher than the other major ions and have a proportionately narrower range of distribution (Stuart and Smedley, 2009). This narrow concentration range for Ca (94 - 144mg L-1) indicates a rapid solution of calcite to its solubility limits. Figure 2.2 shows a plot of major ions distribution in the Chalk aquifer of the Hampshire area
Stuart and Smedley (2009) measured chlorine (Cl) concentration, within the Hampshire area, to be between the ranges of 12.9 - 23.5 mg L-1 with a median of 18 mg L-1 using statistical methods. No obvious spatial trend was observed for Cl concentration. Also, magnesium (Mg) concentrations were low (averaging 1.98mg L-1) as was expected for unconfined Chalk groundwater. This Mg was thought to be most likely reflecting input from reactions with clay minerals in the Palaeogene strata.
Potassium (K) concentration within the Hampshire area (also determined by Stuart and Smedley, 2009), range from below the detection limits of 3.53 mg L-1 having an average value of 1.33mg L-1. This concentration of K as with Mg was suggested to have increased as result of reactions with clay minerals in the overlying Palaeogene sediments or with clay minerals in the Chalk. Concentration of Sulphates is generally low (with an average of 11.8 mg L-1) and has similarities with those found in Dorset Chalk (Shand et al. 2007). Sulphate was suggested to have been derived from atmospheric inputs or from pyrite sources. This is because pyrites are known to be present in hard grounds in the Chalk and also likely to be very much present in Palaeogene deposits.
Nitrate concentrations have value of around 6.5 mg L-1 as nitrogen (N), comparing reasonably well with groundwaters from the Chalk (Shand et al., 2007). NO2-N and NH4-N concentrations are both uniformly low and below detection limits in most areas within the Hampshire (0.001 mg L-1 and 0.066 mg L-1 for NO2-N and NH4-N concentrations respectively). Concentration plot for major ions distribution in the Hampshire Chalk aquifer is shown in figure 2.2.
The measured dissolved oxygen (DO) content, by Stuart and Smedley (2009), are also low (averaging 0.8 mg L-1), similar to that observed from the Dorset Chalk (Edmunds et al., 2002). Soil reactions generally produce DO but are enhanced by pollution input sources (e.g. landfills and slurry pits). The concentration of silicon (Si) is relatively uniform all through the Hampshire region, with ranges of about 4.6 to 9.4 mg L-1. Si in Chalk groundwater can be derived from clay minerals or flint within the Chalk matrix (Stuart and Smedley, 2009).
Figure 2.2: Plot of major ions distribution in the Chalk aquifer of the Hampshire area (adapted from Stuart and Smedley, 2009).
2.4.2 TRACE ELEMENTS
A major trace element controlled by carbonate reaction is strontium (Sr). It varies between 200 and 300 g L-1 in groundwater at outcrops but reaches up to 16 mg L-1 in confined aquifer with close proximity to coastal areas (Adams, 2008). Sr concentration increase can be linked to saline intrusion. A similar enrichment in Sr has been recorded in deep Chalk boreholes elsewhere in Britain by Edmunds et al. (1992). Sr concentration of around 20 mg L-1 has been recorded in pore water from Chalk of 500 m depth at Trunch Norfolk (Bath and Edmunds, 1981).
Stuart and Smedley (2009) studying the concentration of phosphorus (P) in the Hampshire area, observed that P vary widely over the area between a range of 10 - 193 g L-1 with an average of 19 g L-1. In the Chalk groundwater likely sources of P include phosphate minerals, especially those of hard grounds and also exchangeable P from iron oxides. Alternative, while source for P concentration increase can be linked to inorganic and organic fertilizers, no apparent correlation exist between P and other indicators of agricultural pollution such as NO3.
Stuart and Smedley (2009) also estimated the concentration of fluorine (F) to be generally low (averaging 105 g L-1), with the highest values observed in groundwaters emanating from below the Palaeogene cover in the south of the Hampshire area. As with P, F are thought to originate from phosphate mineral (fluoroapatite), present in hard grounds and marl horizons in the Chalk. Bromine (Br) concentrations were observed to be lowest along groundwater divides. Iodine (I) are also low in the central part of the Hampshire area. Of the alkaline metals, lithium (Li) concentration ranges between 0.54 - 1.84 g L-1, with an average value of 0.83 g L-1.
Adams (2008) suggested that the variation of bromine (Br) in groundwater suggest that increases are specifically related to saline intrusion into coastal areas. Groundwater from unconfined aquifers generally does have low concentrations of Br (less than 0.1 mg L-1). They also observed that in unconfined aquifers, Iron (Fe) and Manganese (Mn) concentration are generally low due to oxidising conditions. This they said increased when the groundwater becomes essentially anaerobic under confined conditions, with the greatest increase seen in Fe (in an excess of 1 mg L-1). Both are thought to be derived from natural dissolution under reducing condition, principally from iron and manganese oxides.
Under near neutral conditions, measured concentrations of many trace metals are low in Chalk groundwaters (Admas, 2008). Concentrations of aluminium (Al) are generally below the limit of detection and are typically present in low concentrations in Chalk groundwater due to the alumina-silicate minerals being poorly soluble at circum-neutral pH (Stuart and Smedley, 2009). The maximum detection limits for cadmium (Cd) concentration were below 0.5 g L-1 (which is below detection limit). Cd is most likely to have been adsorbed to oxide surfaces. Zn concentration was found to be highest in Chalk outcrop area around Basingstoke but was generally of an average value of 11.5 g L-1 (Stuart and Smedley, 2008). Zn occurs as a trace element in calcite and clays and has higher concentrations at neutral pH. Because it is a common industrial metal, anthropogenic input may arise when significantly close to urban areas and landfills. Arsenic (As) concentration where also found to be generally low within the Hampshire area (less than 2 g L-1) throughout both Chalk aquifers (unconfined and confined) (Adams, 2008).
2.5 TREATED EFFLUENT DISCHARGE
The Morestead WWTW is the largest return to ground water within the Chalk in England (Facey, 2005). For many years it has been a practice to discharge treated waste water effluents to Chalk at a number of inland sewage treatment works in Hampshire. Subsurface waste water treatment and effluent dispersal refers to the application of partially treated waste water to the subsurface, with infiltration and percolation occurring via the vadose zone or unsaturated zone and subsequently into the saturated zone before finally reaching the underlying groundwater (Siegrist et al., 2000). Depth to the unsaturated zone can potentially affect hydraulic function (hence the purification process) by influencing the soil water content, media surface area, aeration status and the hydraulic retention time (Van Cuyt et al., 2001).
Southern Water Authority in 1975, embarked on a programme of investigation to ascertain the extent and nature of groundwater contamination of the Morestead road WWTW and other of their treatment works. This investigation focused on examining interstitial water quality, chemical microbiological and micro pollutant measurement in pore water and groundwater of the Chalk in Winchester area.
In the Morestead road WWTW, the dry weather flow is 8000 m3 / d. Sewage or waste water is treated by sedimentation and is distributed by gravity to recharge ditches and lagoons (Otterbourne, 1990). Boreholes that supply water to the public are situated on the southern flank of a pericline on top of the outcrop of the Lower and Middle Chalk. The movement of groundwater is generally towards the west (Otterbourne, 1990). Water quality values selected from a number of boreholes within the recharge area and path of effluent flow can be seen in table 2.3.
Examined pore water constituents revealed that contamination within the investigated boreholes extends as far as 85 meters below the water table where hydraulic loading was equivalent to 20 mm/d in utilised recharge areas. Sewage effluent concentration were in the range of 65 to 83% nutrient removal in the Winchester area (Otterbourne, 1990). Also, the oxidation of treated effluent eliminated any processes of nitrogen removal, which was thought to be the likely reason for the loss of dissolved carbon, utilised in the microbiological process of denitrification. The lagoon water source is provided largely by primary effluent recharge but results from Table 2.3, validates pollution occurring from failure of old ditch system near the lagoon (Outterbourne, 1990). Microbial removal such as faecal bacteria and virus is completely eliminated within these sites boundaries.
Table 2.3: Groundwater chemical quality of Winchester site with the average values in milligrammes per litre (adapted from Otterbourne, 1990).
R Area South 1
R Area North 2
Experimental Area 3
R Area boundary 4
2.5.1 EFFLUENT DISPERSAL TECHIQUES IN THE HAMPSHIRE AREA
All treated waste water effluents depart very radically from the ideal pure silt free water used in most research into Chalk aquifer recharge. Primary effluents are usually known to cause the least pollution but always present particular difficulties in dispersal. A number of soakaway methods employed in the Hampshire area are discussed in the subsequent sections below.
18.104.22.168 LAND SPREADING
Land spreading plot laid out in areas underlain by Middle and Lower Chalk of the Morestead road WWTW serving in Winchester, was subjected to primary effluent of poor quality. Results obtained show that the steeper gradients, greater than 1:25, were unsuitable for proposed method of disposal with effluent flowing rapidly downhill and bypassing large areas of land (Otterbourne, 1990). With even shallower gradients, the flow took preferential paths across the plot without soaking into the ground leaving the vegetations clogged with fungus and died. Otterbourne (1990) observed that effluent carbon levels were greater than 15 mg/ l induced sewage fungus growth, concluding that land spreading is not a method that can be used for poorer quality primary effluent. This level of treatment also does not enable the benefits of denitrification to be obtained.
22.214.171.124 EXCAVATED DITCHES
Effluents are discharged into a series of trapezoidal ditches excavated through the top soil deep into weathered Chalk. This method has been applied in the Morestead road WWTW for nearly 100 years (Figure 2.4). In excavated ditches, effluent is discharged into the highest of a series of contoured ditches on the ground of slopes up to 1:10 and moved from the ditch to ditch until all has soaked away (Otterbourne, 1990). Since the treatment at Morestead road is of a primary nature and the settlement tanks are often hydraulically overloaded, the standard even of this treatment is poor. Research into the capacity of the ditches at Morestead road showed that there was decay in performance of all the ditches with time as they became silted up.
Figure 2.4: The Green ditch pattern of Morestead road WWTW allows for treated water to seep back into the Chalk aquifer (adapted from Facey, 2005)
126.96.36.199 SOAKAWAY LAGOONS
The maintenance culture of soakaway lagoons, i.e. effluent must be given conventional primary and secondary settlement before discharge and rotation of the lagoons to allow recovery, make it generally not favoured as well as the hazard present should someone should fall in them. Trials were however taken at Alresford works in 1981 during reconstruction of the French drain for fully treated effluent from the works (Otterbourne, 1990). At the outset the method was satisfactory in dealing with all emerging effluent from the work but rapidly declined as the capacity increased.
188.8.131.52 FRENCH DRAIN SOAKAWAYS
French drains have been the conventional means of effluent disposal mainly from septic tanks for many years. It involves the discharge of effluent to underground porous pipes, making them attractive for effluent disposal. French drains are used at Alresford treatment works. Its general advantage is that they are not visually intrusive and there is much less smell issues than with other methods, aiding there construction not too far away from the population they serve. However, after 10 to 20 years, clogging of the soakaway may occur and have to be replaced even though the effluent has been treated to high quality standards than from a septic tank (Otterbourne, 1990). Reconstruction of the Alresford drainage system in 1981 exposed some flaws in the system in situ. The pores in the gravel packed grounds were completely clogged, preventing the free flow of effluent to the Chalk aquifer (Otterbourne, 1990). Analysis of the pore materials showed that over 80% were organic content likely to have been derived from ineffective secondary sedimentation at the works or from tertiary biological activities taking place within the ground. In many cases it will be the capacity of the clogged gravel pack to transmit water, hence regular resting and rotation of the French drains is now the accepted practice in the Hampshire works engaging in this type of soakaway.
2.6 EFFLUENT DISPOSAL SITES
Site and soil relating factors required in the siting and design process for selecting an appropriate effluent disposal site (Geary, 1987, US EPA, 1980, Siegrist et al., 2000, Dawes and Goonetilleke, 2003), are given below:
topographic consideration, as in the site elevation and slope;
subsurface consideration, including the site soil characteristics and profile, groundwater pathways, depth to the water table, variability and depth to the limiting restrictive soil layer;
land area available for treatment of waste water and effluent disposal;
climatic conditions (rainfall and temperature);
frequency of flooding; and
location and distance to the specific topographic feature (waterways and wells).
For long term acceptance of effluent, it is necessary that these factors are put to consideration when designing and selecting a suitable site for effluent disposal systems. In selecting a suitable disposal site of effluent discharge from a WWTW, understanding of the reservoirs ability to accept, treat and disperse the effluent is crucial. This is because the heterogeneous nature of the soil makes assessment of a single soil parameter incomprehensively suitable (Diack and Stott, 2001). Therefore there is the need for more scientific rigorous procedure when assessing soil suitability for treated effluent disposal and removal of important pollutant. Some common subsurface effluent disposal sites include;
They are constructed as shallow excavations with a perforated pipe lay over gravel to enable the even distribution of applied effluent. They are most suitable where the soils are moderately permeable and remain unsaturated for reasonable depth below the surface (Goonetilleke et al., 1999).
It differs from the trench in that more than one effluent distribution pipe is provided over a much wider area. This distribution network allows for smaller effective area set-up necessary for effluent distribution than those required for the trench system, making the bed more suitable foe restricted site areas (Carroll, 2005).
Other subsurface disposal sites that are alternatively available for use in areas where the trench and bed systems are considered inappropriate for providing effective effluent dispersal, include the mound system (designed to overcome of dispersing partially treated effluent in areas of low soil permeability or high ground water table or existing cracked bedrock) and evapotranspiration systems (utilises climatic conditions to evaporate effluent from shallow trenches combined with transpiration through the use of vegetation planted specifically for the available water and nutrient utilisation).
2.7 HYDROGRAPH OF CHALK AQUIFERS
Neuman (2007) analysed and combined sixteen boreholes of depths between 11.7 to 183.8 m to generate one master hydrograph for the Chalk in Winchester and South dams (Figure 2.5 and 2.6). The observed water levels (Neumann, 2007) show seasonality and an underlying trend of relatively low levels around 1973 and 1976 to 1992 - 1997. However, extreme high events are seen during the winters of 1993/1994 and 2000/2001 (Neumann, 2007). This master hydrograph was established from March 1953 to October, 2004 (Neumann).
Figure 2.5: Master Hydrograph for the Chalk in Hampshire and Wiltshire area (adapted from Neumann, 2004).
Figure 2.6: Water level frequency categories per monthly period (adapted from Neumann, 2004).
CHAPTER THREE: FIELD SAMPLING AND ANALYTICAL METHODOLOGY
3.1 BOREHOLE LOCATIONS
Data chosen from twelve (12) borehole (MWE02, MWE05, MWE06, MWE07, MWE08, MWE09, MWE11, MWE12, MWE13, MWE14, MWE15 and MWE16) locations are used for this research. The boreholes are in Winchester area, Hampshire region. The borehole locations (see Figure 1.2, pp 6) are presented in Table 3.1 below.
Table 3.1: Position coordinates for each borehole studied within the Morestead WWTW site.
BOREHOLE NAME (MWEXY)
3.2 FIELD SAMPLING TECHNIQUE
Vital to the success of this research was the obtaining of core samples. To fix the research area within a stratigraphic context, core samples with unaltered Chalk and obtained porewater, for laboratory testing, were collected and analysed. The percussion drilling (U100) technique (a rugged, cheap sampler that produces core samples in most British clays typically heavily overconsolidated) was used to intermittently core the new Alresford site.
For the Morestead road WWTW, air-flush rotary drilling technique was employed instead of percussion drilling to recover U100 samples, since it has been observed that the rotary coring method is far superior in the Chalk. The agreement by Southern Water to this additional cost was to give more reliable results. The Morestead Road was rotary cored, using a mist flush method. Obtained cores were transferred to a thin - walled PVC tube, which were waxed and capped (Munn, 2008). This approach gave a near continuous recovery and eventually a much better understanding of the site. On site, the cores were stored in refrigerated containers before they were later transferred in a refrigerated lorry to the refrigerated core store at the University of Brighton, Cockcroft Building. The cores experienced a steady temperature of 2 - 80C after they were extracted (Munn, 2008).
3.3 REASON FOR CORE SAMPLES REFRIGERATION
Of utmost necessity is the assurance that the pore water extracted from the core samples reflects as closely as possible in situ pore water that were actually in the Formation. Several processes grossly alter pore water chemistry prior to analysis. Some of these are summarised as follows below:
diffusion of dissolved gases, such as ammonia, oxides of nitrogen and semi - volatile organic compounds, down chemical gradients, as a function of temperature and partial pressures. Thus, reduction of the core temperature causes a decrease in the rate at which water will evaporate (Bohren, 1987).
contamination from the container or packing materials, e.g. newspaper used to fill voids in the core boxes / U100 tubes.
An assumption of remediation of most, if not all, of the target analytes was made due to bioremediation, at the beginning of the study (Green et al., 2001). This was because bacteria are known to naturally metabolise a range of compounds (Stembal et al., 2005, Benedict and Carlson, 1971, Topping, 1987) in order to obtain energy for respiration. Hence, reduction of the temperature of the cores reduces drastically the risk of samples being altered by biochemical processes, ensuring that the pore water samples obtained are represent in situ Formation.
3.4 ANALYTICAL TECHNIQUES
3.4.1 CORE LOGGING AND POREWATER EXTRACTION TECHNIQUE
The initial research phase required the extraction of pore water from Chalk cores extracted from the Morestead road site. This required that cross contamination of samples be kept at an absolute minimum, and that maximum pore water recovery was achieved. In addition to the above, it was deemed desirable that the obtained pore water underwent as little chemical alteration during centrifugation process and subsequent storage prior to analysis.
3.4.2 CORE LOGGING AND STORAGE TECHNIQUE
Before they were then transferred to the soils laboratory for logging, the collected cores were stored at temperatures between 2 and 80C. Heavy polythene bags were used to cover the surface upon which logging of the samples was to be carried out. These were renewed for each logged sample. After logging was successfully carried out, all implements used for logging were cleaned using a propriety combined detergent and bactericide. The core logging was carried out by Professor Rory Mortimore, due to his expertise in this field. A geological hammer was then used by Munn (2008) to render the Chalk down into pieces of approximately 5 mm diameter ready for centrifugation
3.5 PORE WATER EXTRACTION TECHNIQUE
Several approaches exist for extracting pore water from soils and porous rocks, however no single methodology is appropriate to all applications. The choice of a method will hence depend on the particular aim for which the study was carried out. Therefore, description of the methodology employed is very important as well as stating the assumptions made. Generally, most field sampling methods have been employed to interpret both the static and dynamic perspective of Chalk pore water chemistry, without much relevance paid to the Chalk water being sampled and its chemical reactivity (Wolt, 1994).
Core sample pore water can be extracted by either a field-based approach (such as tension samplers, monolith, and passive samplers etc - all referred to as lysimeters) or laboratory-based methods (such as rhizonâ„¢ samplers, centrifugation, and pressure filtering). For this study we focus on pore water extraction using centrifugation technique. This is because centrifugation can be used to fractionate the pore water by selecting several centrifugation rates. Therefore, when increasing the centrifugal speed, and therefore the relative centrifugal force (RCF) value, during several stages of soil centrifugation, less available water may gradually be released and collected (Tyler, 2000).
The use of centrifuge method to extract water from porous media has a long history. It was originally developed to establish "moisture equivalent" of a soil, i.e. the moisture content of a sample after the excess water has been reduced by centrifugation and brought to a state of capillary equilibrium with the applied force (Nuclear Energy Agency, 2000). Centrifuge technique relies on the difference in pressure developed across a sample exceeding the capillary tension holding the water in the pores.
Chalk cores centrifugation was undertaken by Munn (2008) in order to extract pore water without severely altering the pore water chemistry. In order to limit gaseous exchange, both of the water itself and of any volatile or semi-volatile contaminants, refrigerated centrifuges were utilised. He used Rotanta 460R centrifuge in which the rotor arm and bucket are in Computer Numerical Control (CNC) machined units. These made them able to withstand the cyclic loadings that could be applied to them during centrifugation.
3.5.2 CENTRIFUGE SET - UP
In practise, it is inadvisable to reduce the temperature of a centrifuge below 120C as icing could tend to collect on the centrifuge buckets and rotor arm. This could potentially unbalance the rotor - arm during centrifugation, leading to unnecessary wear (Munn, 2008). Icing is believed to occur due to reduced air - pressure in the centrifuge chamber during centrifugation caused by the movement of the rotor arms. The apparatus design developed by Nigel Munn for collecting pore water, separated from the bulk matrix during centrifugation, is shown in Figure 3.1. This head-space arrangement was found to be effective for the successful completion of the pore water extraction programme. Approximately 400 g of broken Chalk samples was placed into each container. The entire arrangement was then loaded into the centrifuge buckets. Typically, the centrifuge was programmed to spin at 2500rpm for a duration of one hour at 120C. The diagram of the head - space equipment is show in Figure 3.1.
Figure 3.1: Diagram of Head - space equipment showing its assemblage (adapted from Munn, 2008).
3.6 MEASUREMENT OF IONIC AND DETERMINAND CONCENTRATIONS
Inorganic cations, trace elements and sulphates were measured by inductively-coupled plasma optical-emission spectrometry (ICP-OES) and anion species by automated colorimetry at Southern Water's laboratories. Sample preparation and analysis were carried out at the same laboratory. Ionic balances for the analyses were within appreciable limits and the precision for the trace elements were verified (Munn, 2008). pH was analysed by ion selective electrode (ISE).
Concentration of ions and determinands in the final effluent were evaluated from analytical results obtained from Southern Waters laboratory.
All data handling and plots were done using Microsoft office Excel 2010 spreadsheet.
CHAPTER FOUR: RESULTS
4.1 MORESTEAD ROAD BOREHOLE CORES
Core logging was carried out by Prof. Rory Mortimore (Munn, 2008). The rotary core logging generated the following information:
litho- and biostratigraphical data required to place the site and ground profile in its correct stratigraphical and structural geology setting. Also, these data will aid in assessment of the overall Chalk geology in relation to ground permeability.
The rotary method of drilling provided excellent representative cores that allowed for all lithological features to be identified. Stratigraphic representation of encountered Formations during the rotary drilling process is presented in the subsequent section.
4.2 STRATIGAPHIC SEQUENCE
Several Formations where encountered during the drilling of the Chalk at Morestead Road site. These include, Holywell, New Pit and Zig Zag. Stratigraphic representation of the Chalk at the Morestead Road site, from each borehole as a function of depth interval, is presented in Table 4.1 below. Also in figure 4.2, a detailed stratigraphic succession of drilled Chalk from borehole MWE05 is presented (Munn, 2008).
Drilled Chalk from borehole MWE05 occurred from 1.5 to 20.0 mm depth below ground level (BGL). Identification of particular marker beds were less pronounced as this borehole was mostly in the Zig Zag Chalk Formation (Munn, 2008).
Table 4.1: Stratigraphic Distribution of the Chalk at Morestead Road WWTW site
C:\Users\Christopher\Desktop\Liman Data\Data for Liman\BH MWE05.jpg
Figure 4.1: Drilled Chalk from Borehole MWE05 of the Morestead Road WWTW site, showing encountered Formations and beds (adapted from Munn, 2008).
4.3 VARIATION - TIME GRAPH
In an attempt to determine potential source and rate of recharge (in both summer and winter months) from the surrounding vicinity of the boreholes at Morestead Road WWTW site, several plots where made of measured water level (using in situ pressure loggers with recordings at 15 minutes interval) as function of averaged date and time, for the same period. For these study, data from 12 borehole drilled through the Chalk at Morestead Road WWTW site where considered and the discussion of observed findings is to be presented in Chapter five.
An average temperature of about 15.560C was measured. It can be seen (Figure 4.2) that the water level depth is between 36.448 to 43.036 mAOD. It is at a minimum during winter months (November to December) than in summer where a peak of 43.036 mAOD is observed for Early August.
Figure 4.2: Water depth variation - Time plot for borehole MWE02
An average temperature of about 10.830C was measured. It can be seen (Figure 4.3) that the water level depth is between 38.617 to 43.532 mAOD. It is at a minimum during winter months (November to December) than in summer where a peak of 41.835 mAOD can be observed for Early August.
Figure 4:3: Water depth variation - Time plot for borehole MWE05
An average temperature of about 10.680C was measured. Water depth ranges between 39.574 to 47.888 mAOD. An almost evenly distributed water depth variation can be seen (Figure 4.4) from the plot.
Figure 4.4: Water Depth Variation - Time plot for Borehole MWE06
An average temperature of about 10.870C was measured. Water depth ranges between 33.132 to 35.179 mAOD. Also, an almost evenly distributed water depth variation can be seen (Figure 4.5) but not as defined as with MWE06, from the plot. A zig zag arrangement is easily identified.
Figure 4.5: Water Depth Variation - Time plot for Borehole MWE07
An average temperature of about 11.270C was measured. Water depth ranges between 33.571 to 36.863 mAOD. Water depth variation is at a minimum during winter months (November to December) than in summer and autumn with a peak of 36.863 mAOD observed in Early August (Figure 4.6).
Figure 4.6: Water Depth Variation - Time plot for Borehole MWE08
An average temperature of about 12.900C was measured. Water depth ranges between 34.588 to 38.842 mAOD. Water depth variation is at a minimum during winter months (November to December) than in summer and autumn with a peak of 38.842 mAOD observed in Early August (Figure 4.7). The plot is similar to MWE08.
Figure 4.7: Water Depth Variation - Time plot for Borehole MWE09
An average temperature of about 12.900C was measured. Water depth ranges between 37.721 to 43.122 mAOD. Water depth variation is at a minimum during winter months (November to December) than in summer and autumn with a peak of 43.122 mAOD observed in Early August (Figure 4.8). From September to October the variation is relatively constant before a steep decline occurs until the end of November and early December.
Figure 4.8: Water Depth Variation - Time plot for Borehole MWE13
An average temperature of about 11.500C was measured. Water depth ranges between 38.380 to 43.979 mAOD. Water depth variation is at a minimum during winter months (November to December) than in summer and autumn with a peak of 43.979 mAOD observed in Early August (Figure 4.9). From September to October the variation is relatively constant before a steep decline occurs until the end of November, where few peaks and falls are observed before another decline to lowest in December.
Figure 4.9: Water Depth Variation - Time plot for Borehole MWE14
An average temperature of about 8.290C was measured. Water depth ranges between 37.679 to 39.512 mAOD. Water level experiences a decline from July to Early August before a sharp increase is observed to a peak value of 39.512 mAOD. Thereafter, a steep decline is occurs to early September before a steady and constant value of 37.922 mAOD stretches the entire distribution to December (Figure 4.10).
Figure 4.10: Water Depth Variation - Time plot for Borehole MWE15
4.3.1 CORRELATION BETWEEN VARIATIONS - TIME GRAPH
Using the average water level values per day, four plot where made to compare water level variations within these borehole dug through the Chalk of Morestead Road site.
Between MWE02 and MWE05
Superimposing MWE05 plot on MWE02, an almost similar distribution in water level variance is observed. However, between October and November a clear-cut difference is observed (Figure 4.11).
Figure 4.11: Variance in Depth against Time correlation between MWE02 and MWE05
Between MWE06 and MWE07
Superimposing MWE07 plot on MWE06, a similar distribution in water level variance is observed (Figure 4.12).
Figure 4.12: Variance in Depth against Time correlation between MWE06 and MWE07
Between MWE08 and MWE09
A superimposed plot correlating water level variation in borehole MWE08 and MWE09 show an almost similar distribution between the measured data (Figure 4.13).
Figure 4.13: Variance in Depth against Time correlation between MWE08 and MWE09
Between MWE13 and MWE14
A superimposed plot correlating water level variation in borehole MWE13 and MWE14 show an initial variation, with a downward peak for borehole MWE14 in mid-August, before a close similar distribution can be observed from the measured data (Figure 4.14).
Figure 4.14: Variance in Depth against Time correlation between MWE13 and MWE14
4.4 EFFLUENT DICHARGE CONCENTRATION WITH DEPTH
Chemical analysis was conducted on porewater samples gotten from core logs of the Chalk at Morestead Road WWTW site boreholes, as a function of their respective depth, for the estimation of both major, trace and determinants concentration. Five plots of concentration variation against depth are made for boreholes MWE02, MWE07, MWE09, MWE13 and MWE15. The major elements analysed are Ammonia, Carbon (in Total Organic Carbon), Chloride, Nitrate and Nitrite. These plots are presented in Figure 4.15, 4.16, 4.17, 4.18 and 4.19, respectively. Chapter five discusses the plausible source, effect and potential effect over the long run of these concentrations to groundwater in the Chalk of the study area.
Figure 4.15: Effluent discharge concentration - Depth plot for Borehole MWE02
Figure 4.16: Effluent discharge concentration - Depth plot for Borehole MWE07
Figure 4.17: Effluent discharge concentration - Depth plot for Borehole MWE09
Figure 4.18: Effluent discharge concentration - Depth plot for Borehole MWE13
Figure 4.19: Effluent discharge concentration - Depth plot for Borehole MWE15