Recharge: the Key to Groundwater Pollution and Aquifer Vulnerability

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Recharge: the key to groundwater pollution and aquifer vulnerability

N. S. ROBINS

British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, 03210 8BB, UK

Abstract: Recharge is pivotal to understanding the processes by which groundwater pollution can occur. It is implicit in the classification of aquifer units according to their vulnerability to pollution. The management of both groundwater resources and of individual groundwater sources cannot sensibly be undertaken without some knowledge of recharge: its quantity, its seasonality and, above all, the different routes through the sub-soil and the unsaturated zone by which it can occur. However, current estimates of recharge, other than on a research site basis, may be poor, both in the UK and overseas. This volume provides a review of current research into these issues; this introductory paper attempts to highlight the thread throughout all of this work which collectively provides the basic information in support of the current and future management of groundwater resources and sources.

This volume serves to highlight current develop- introductory paper attempts to highlight some ments in the assessment of groundwater pollu- of the issues pertaining to recharge estimation, tion, aquifer recharge and vulnerability, but and to the measurement and modelling of more particularly it illustrates the breadth of recharge; to the complex relationship between scientific enquiry that is currently being applied recharge, groundwater pollution and aquifer vul- to this complex, but topical, field of study. More- nerability; and to the importance of the recharge over, the volume demonstrates that the key to estimate. understanding many aspects of groundwater pollution lies alongside the reliable estimation of recharge, and also implicitly in the definition of Recharge estimation: the pitfalls aquifer vulnerability. To this end, a great deal of effort is now being put into investigating the Traditional estimates of recharge have depended processes that control recharge and to the appli- on the closure of the water balance for a given cation of contemporary findings. catchment or hydrogeological unit. Measure- Various groundwater management tools have ment of rainfall, evapotranspiration and run- been, and are being put into place by the Envir- off provides an indirect estimate of infiltration onment Agency in the UK. Many of these were by remainder. However, history has shown that first described in Policy and Practice for the Pro- each new method of determining infiltration tection of Groundwater (NRA 1992), a document and recharge based on secondary observations which has since been emulated by Scotland and (e.g. climatic and hydrological data) falls ulti- will shortly also be followed by Northern Ire- mately into part disfavour. land. This policy document clearly recognizes Calculation of evaporation from open water the important role of recharge and acknowledges using an evaporation pan, and the determination potential areas in which knowledge and data are of the same from meteorological data (Penman currently inadequate for the rigid application of 1948) was overtaken by the estimation of tran- the strategy. Data are least available in areas of spiration for particular types of vegetation upland Britain, including Scotland and Northern (Penman 1963). The general expression for tran- Ireland, where information has not been col- spiration depends on the sum of the available lected so assiduously by the developers of the energy and an aerodynamic term, the latter a lesser aquifers as it has been of necessity by work- function of wind-speed and humidity. Nowadays ers on the major aquifers such as the Chalk, the this technique, modified by Monteith (1965), Triassic sandstones and the Jurassic limestones forms the basis of the Morecs 40 by 40 km of England. Lack of detailed knowledge and block averages for the UK. In the absence of understanding, particularly of relevant pro- site-specific information, these are generally the cesses, is apparent from much of the work only available data. described in this volume; a great deal of research Estimates of run-off using low surface water has yet to be undertaken, on both a generic and a flow data (Gustard et al. 1992), although widely site-specific basis. available, provide little insight into the nature This volume concentrates on the key role of of the baseflow contribution that is actually recharge in groundwater management. This maintaining the low flow. They do, however,

ROBINS, N. S. 1998. Recharge: the key to groundwater pollution and aquifer vulnerability. In: ROBINS,N. S. (ed.) Groundwater Pollution, Aquifer Recharge and Vulnerability. Geological Society, London, Special Publications, 130, 1-5 Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

2 N.S. ROBINS allow regional estimates of recharge to be made Environment Service of Northern Ireland. based on geolines for aquifers and non-aquifers, Besides, the role of till in inhibiting recharge is and low flow indices (Robins 1990). The hydrau- locally zero wherever it is removed or effectively lic relationship between surface and groundwater removed, for example in a quarry or a railway at any given point in a stream or river bed has cutting, or indeed where the source is below the first to be defined before low flow data can be protective cover, such as a leaking sewer set at used rationally. That being so, data are available the base of the till. The published vulnerability for large parts of the UK. maps identify 5 m as the critical thickness of till Even a simple parameter like rainfall is not as a protecting cover to an aquifer, but acknowl- easily assimilated into a recharge calculation. edge that till integrity is crucial but generally The rainfall event, its intensity and duration, unknown (Palmer et al. 1995). are as critical as the existing soil moisture The actual mechanism by which recharge status. Furthermore, microclimatic variation occurs is also very important. Price et al. (1993) may provide significant difficulties in rational argue that the Chalk has three sets of porosity: extrapolation of data even across a small catch- microfissures which allow rapid egress of storm ment area. Clearly, accurate determination of water; microfissures which promote storage and recharge must be done on a site-specific basis transport; and the intergranular matrix of the using a direct measurement such as a lysimeter Chalk blocks. Jones & Cooper (1998) demon- (see Jones & Cooper 1998) or using soil moisture strate that 30% of the potential recharge within measurement and event modelling (see Blackie et the Fleam Dyke lysimeter in Cambridgeshire al. 1998). These techniques are, however, expen- takes place within fractures. Many other aquifers sive. It may be that the way forward is through offer dual porosity, the Triassic sandstones are regional digital hydrogeological models which largely fractured and may also contain dilated will assist in establishing overall renewable bedding planes sufficient to offer rapid egress to resource values for a given aquifer unit rather percolating water. At the extreme end of the than point estimates of actual recharge. scale, karstic limestone provides selective, but A further significant problem arises in Britain potentially more rapid egress, transport and dis- through the existence of glacial till, which over- charge, in this case between sinks and risings, but lies bedrock over a significant part of upland with almost no storage (Daly & Warren 1998). Britain and much of lowland Britain north of The role of the soil zone is critical in the infil- the Severn and the Wash, and the Clay-with- tration process. The texture and grain size Flint that occurs on some of the Chalk outcrop. together control the ability of a soil to retain or It is often considered that the existence of till to drain soil moisture, both saturated and unsa- reduces potential recharge to an underlying aqui- turated. Palmer & Lewis (1998) explain in some fer by 30%, and such a rule of thumb has served, detail how the structure and density of a soil for the most part, reasonably satisfactorily. But influence the progress of permeating fluids, and what of the nature of the till itself?. There is a con- how the presence of clay minerals and organic siderable difference between the thick clay till matter influence the so-called leaching potential cover of the Fylde, and the often sandy till with of a soil (the ability of a soil to transmit non-con- pebble lenses that occurs in many areas else- servative pollutants). Cation exchange also where. The thick till deposits of North Yorkshire assists degradation and attenuation of some pol- and Humberside clearly inhibit all recharge, but lutants within the soil zone. how effective are the thin veneers of till and Estimation of recharge is, therefore, fraught Clay-with-Flint? with difficulty, a theme which is expanded by Mapping geologists have always concentrated Simmers (1998). Recharge can be measured on bedrock mapping; at best till is depicted as a directly and accurately on a site-specific basis, pale blue wash, with little regard for its integrity but extrapolation of such data, either in time at or thickness. Recent mapping of superficial the measured site or spatially away from the strata, such as that reported by Browne & measured site, is difficult, not least because the McMillan (1991) for the Glasgow area and else- prevailing conditions do not remain constant where in Scotland, has attempted to provide the and because the principal recharge mechanism detail with which recharge distribution can be may differ from one location to another. attempted, but areas so far covered are few. There is additional uncertainty in estimating For the most part, site-specific investigations recharge in a semi-arid climate where recharge are necessary in areas where low-conductivity events need not necessarily occur on an annual till is believed to be present. This is clearly basis and may even be complicated by urbaniza- advised on the aquifer vulnerability maps of the tion and man-made lagoons (Alderwish & Dot- Environment Agency, Scottish Office and the tridge 1998). 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RECHARGE 3

Groundwater pollution and recharge related to recharge, or more strictly to the acces- sibility of the aquifer to recharge, independent of The obvious difficulties of measuring recharge effective rainfall. There are many different sys- and even of understanding the prevailing tems for deriving a working model and hence a recharge processes, detract considerably from map of the degrees of vulnerability (Vrba & studies of groundwater pollution in the unsatu- Zoporozec 1994) and this is amply illustrated rated zone. Recharge has two direct effects on by comparing work in England and Wales dispersed pollutants: the greater the recharge (Palmer & Lewis 1998) and in the Republic of the greater is the potential for dilution, but the Ireland (Daly & Warren 1998). more recharge the more efficient the transport The two national strategies start from quite of pollutant from surface to water-table. In the different vantages. The former developed from former case, the occurrence of nitrate in uncon- a mapping programme designed to delineate fined aquifers correlates directly to rainfall: the vulnerability of aquifers to pollution from nitrate concentrations in groundwater are least agricultural fertilizer, i.e. from diffuse pollution because of dilution in the higher rainfall areas caused by the leaching of excess nitrate applied of Northern Ireland and west England than in to the ground (Robins et al. 1994). These maps the low rainfall areas of eastern England, notably have now given way to the current series of aqui- East Anglia, Scotland, and even Jersey. fer vulnerability maps, which provide for any Whereas calculations of recharge derived from reasonably conservative diffuse pollutant, regional models calibrated with long-term rain- including nitrate, but also for any point-source fall and hydrograph responses enable a renew- pollutant discharging at the surface to produce able resource estimate to be derived, these saturated conditions at or near the surface techniques have little to offer pollution studies. (Adams & Foster 1991). This system, therefore, The question of whether our understanding of places emphasis on the role of the topsoil as an recharge mechanisms is adequate to characterize inhibitor to the downward migration of diffuse flow rates and pathways in the phreatic aquifer sources of pollution. However, the main source recharge process and in transfer to semi-confined of pollution in the Republic of Ireland is not dif- aquifers is taken up by Foster (1998). This ques- fuse or surface point sources, but rather the tion has obvious implications for groundwater point-source pollution posed by numerous pollution protection policies in the UK and else- underground septic tanks. These are designed where. The recharge mechanism is also critical; to overflow at a depth of between one and two the role of by-pass routes and the occurrence of metres below ground surface; consequently the fractures and other preferential flow paths clearly topsoil does not feature in the equation, which influence pollutant transport potential, although in Ireland emphasizes permeability and thickness the specific issues pertaining to fractured rocks of unsaturated zone strata. are not described in the policy document for Eng- Other significant differences are that the UK land and Wales (NRA 1992). mapping programme only classifies the main or Management of potential sources of pollution regionally important aquifers (major and minor also forms an important component of ground- aquifers). Not all the aquifers in the Republic water protection strategy. This too hinges on of Ireland fall into these categories and it understanding the water balance at a particular would be an inappropriate method to apply, site (Sears 1998). Conversely, the identification where all aquifers - regionally important, locally of the source of pollution is critical to remedia- important and poor aquifers - are classified. The tion. Green et al. (1998) used nitrogen isotopes Irish system is thus a scheme to map the vulner- to prove that elevated nitrate concentrations in ability of all groundwater and not just the more Jersey groundwater derive principally from agri- important aquifers. The other significant differ- cultural fertilizer rather than septic tanks and ence is that whereas the aquifer vulnerability soakaways. classification is used separately from source protection zonation to define control measures in the UK, vulnerability and source protection together Aquifer vulnerability and recharge define control measures in a single matrix in the strategy recommended for the Republic of Ire- In Britain, aquifer vulnerability is determined land. according to the nature of the overlying soil The problems of dealing with karstic lime- cover; the presence and nature of the drift; the stones are actively being considered in Ireland. nature of strata; and the thickness of the unsatu- Recommended policy in the Republic of Ireland rated zone (NRA 1992). Collectively, these four is that each sink be mapped with a 30 m radius variables provide a function which is implicitly circle which is designated 'extreme vulnerability', Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

4 N.S. ROBINS and this is extended to include the whole of the wick University in April 1996, and from a workshop surface water catchment that drains into it. The on 'Aquifer Vulnerability' which was presented at the karst is clearly an area that will tax groundwater meeting by the Environment Agency. This paper is managers for a long time to come; the Environ- published by permission of the Director, British Geolo- gical Survey (NERC). ment Agency is now recognizing the importance of karst in England and Wales, particularly with regard to Mendip and other similar areas, and contemporary literature (e.g. MacDonald et al. 1998) suggests that the English Chalk is in part References karstic. ADAMS, B. & FOSTER, S. S. D. 1991. National groundwater protection policy: hydrogeological criteria for division of the land surface area. National Rivers Authority, R&D Note 6. Groundwater protection --& MACDONALD, A. M. 1998. Aquifer susceptibil- ity to side-effects of groundwater exploitation. The key to a successful groundwater manage- This volume. ment policy is an understanding of recharge ALDERWISH, A. M. & DOTTRIDGE, J. 1998. Recharge and recharge processes. In an English context components in a semi-arid area: the Sana'a this depends heavily on good quality Quaternary Basin, Yemen. This volume. BLACKIE, J. R., HOUGHTON-CARR, H. A., MCCARTNEY, mapping, but this is not yet universally available. M. P. & MOORES, J. P. 1998. Estimation of ground- Burgess & Fletcher (1998) describe in detail the water recharge on Jersey. This volume. regulatory policy applied for the delineation of BROWNE, M. A. E.& MCMILLAN, A. A. 1991. British groundwater source protection zones. Together Geological Survey thematic geology maps of Qua- with aquifer vulnerability zoning, be it the Eng- ternary deposits in Scotland. In: FORSTER,A., CUL- lish method or the Irish method, these zones SHAW, M. G., CRIPPS, J. C., LITTLE,J. A. & MooN, enable suitable control measures to be applied C. F. (eds), Quaternary Engineering Geology. Geo- in order to safeguard groundwater from pollu- logical Society, London, Special Publication, 7, tion. 511-518. BURGESS, D. I. & FLETCHER, S. W. 1998. Methods used All groundwater protection policies must be to delineate groundwater source protection zones reviewed from time to time. As technology in England and Wales. This volume. advances, and as understanding of groundwater DALY, D. & WARREN, W. P. 1998. Mapping ground- processes allows, the existing policies will be water vulnerability: the Irish perspective. This adapted to provide the best possible and most volume. suitable policies for resource and source manage- FOSTER, S. S. D. 1998. Groundwater recharge and pol- ment. To this end, the discussion on karstifica- lution vulnerability of British aquifers: a critical tion in the Chalk (MacDonald et al. 1998) and overview. This volume. the discussion of whether the KeUaways Sand is GREEN, A. R., FEAST, N. A., HISCOCK, K. M. & DENNIS, an aquifer containing groundwater that should P. F. 1998. Identification of the source and fate of nitrate contamination of the Jersey be controlled (Mather et al. 1998) provide lines bedrock aquifer using stable nitrogen isotopes. of enquiry to be pursued by the policy makers. This volume. Perhaps the concept of mapping the susceptibil- CUSTARD, A., BULLOCK, A. & DIXON, J. M. 1992. Low ity of aquifers to over-exploitation may become flow estimation in the United Kingdom. Institute an important future management tool (Adams of Hydrology Report No. 108. & MacDonald 1998) although it is most likely JONES, H. K. & COOPER, J. D. 1998. Water transport to be applicable in regions of new and rapid through the unsaturated zone in the Middle groundwater development. Chalk: a case study from Fleam Dyke lysimeter. In conclusion, the current policy and strategy This volume. MACDONALD, A. M., BREWERTON,L. J. & ALLEN, D. J. for groundwater protection in the UK (NRA 1998. Evidence for rapid groundwater flow and 1992), and the policy now recommended in the karst-type behaviour in the Chalk of Southern Republic of Ireland, represent the state of the England. This volume. art and provide the best possible and best practi- MATHER, J. D., HALLIDAY,D. & JOSEPH, J. B. 1998. Is all cal means of managing both resources and groundwater worth protecting? The example of sources at the moment. the Kellaways Sand. This volume. MONTEITH, J. L. 1965. Evaporation and environment. This volume arises from a symposium on 'Ground- In." The State and Movement of Water in Living water Pollution, Aquifer Recharge and Vulnerability' Organisms. Proceedings of the 19th Symposium which was held as part of the first biennial meeting of of the Society of Experimental Biology, Swansea, the Geological Society on 'Applied Geology' at War- 1964, 19, 205-234. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

RECHARGE 5

NRA. 1992. Policy and Practice for the Protection of of the Chalk of North-West Europe, Clarendon, Groundwater. National Rivers Authority, Bristol. Oxford, 35-58. PALMER, R. C. & LEWIS, M. A. 1998. Assessment of ROBINS, N. S. 1990. The Hydrogeology of Scotland. groundwater vulnerability in England and Wales. HMSO, London. This volume. , ADAMS, B., FOSTER, S. S. D. & Palmer, R. 1994. , HOLMAN, I. P., ROBINS, N. S. & LEWIS, M. A. Groundwater vulnerability mapping: the British 1995. Guide to Groundwater Vulnerability Mapping perspective. HydrogOologie, 3, 35-42. in England and Wales. HMSO, London. SEARS, R. 1998. The British Nuclear Fuel's Drigg low- PENMAN, H. L. 1948. Natural evaporation from open level waste site characterization programme. This water, bare soil and grass. Proceedings of the volume. Royal Society of London (A), 193, 120-145. SIMMERS, I. 1998. Groundwater recharge: an overview -- 1963. Vegetation and Hydrology. Commonwealth of estimation 'problems' and recent Agricultural Bureau, Soils, Technical Communi- developments. This volume. cation 53. VRBA, J. & ZOPOROZEr A. (eds) 1994. Guidebook on PRICE, M., DOWNING, R. A. & EDMUNDS, W. M. 1993. Mapping Groundwater Vulnerability. International The Chalk as an aquifer. In: DOWNINg, R. A., Association of Hydrologists, Hannover, Inter- PRICE, M. & JONES, G. P. (eds) The Hydrogeology national Contributions to Hydrogeology, 16.