The list of primary indicators in Table 2 has been reorganised 74 Proceedings
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UNESCO / NWRC /ACSAD W or s o k h p I on “Wadi Hydmhgy ” and “Groundwatrr Protection ”Indicators of Groundwater Quality
Table 2. Recommended indicators
of
rapid environmental change in groundwater systems.PROCESSES PRIMARY
(Table 3) on the basis of the issues and/or problems that are considered of greatest significance and concern. Six of the twelve issues are given the highest priority. It is then possible to select a total of 8 indicators which, if used singularly or the monitoring of radioactivity but this cannot be considered as a routine measurement like the others.
Measurement of trends in the main eight parameters are likely to give important indications of changes in the groundwater, brought about directly or indirectly by man.
Three of these (HCO,, Cl, NO,) may also be used to study historical trends in the evolution of groundwater quality since their analysis has been relatively robust over this timescale. The indicators chosen here are very similar to those recommended by the European Community (water level, conductivity, pH, redox, NO,, DOC, chlorocarbons, metals) as being amenable to automated monitoring with current or new technology (CEC 1993).
Implementation of measurement programmes for groundwater indicators need to be harmonised with individual national programmes. Thus the range of key indicators has also been kept small so as to be compatible with many existing national programmes. In some poorer countries, it might be easier to find support for a basic programme of monitoring rather than a sophisticated network.
The secondary indicators are considered desirable as a means of corroboration. They provide a means of understanding the nature of environmental change using supporting geochemical studies, either through monitoring or in special research studies. If an aquifer system is well understood then, a very simple monitoring programme may be all that is required to observe it.
There already exist a number of other national and international programmes designed to monitor changes in hydrological and biological systems. Groundwater forms an integral part of the hydrosphere. The indicators proposed here are designed to fit in with other proposed schemes.
The groundwater environment may be seen as the output from the surface or from the soil zone. Over the medium to
long term, groundwater is also a net contributor to surface waters, with any pollution source likely to persist for decades if not centuries. Good collaboration must thus be established between geoscientists and others involved in monitoring the hydrosphere.
Several countries have already developed programmes for groundwater monitoring.
These programmes are designed with the goal of ensuring the protection of groundwater for public supplies. The level of activity is related to the size of country, the complexity of its hydrogeology, the importance of groundwater relative to surface water and, not least, the affordable costs. The existing monitoring programmes include some but not all of the parameters proposed as geoindicators.
The Dutch monitoring network is widely regarded as one of the most comprehensive national programmes. The design of a centralised monitoring system is relatively easy for Holland, which has aquifers composed, almost exclusively, of unconsolidated sediments. A purpose-built network of observation boreholes with both shallow and deep screens is combined with the public supply network to give a very detailed and three-dimensional network for sampling of groundwaters (van Duijvenboden 1987;
Frapporti et al 1993). A similar approach is adopted in Denmark (Korkman 1987), which also relies heavily on unconsolidated aquifers for water supplies. In other
European countries, the national groundwater monitoring strategy is less clear and/or under review. The hydrogeology of UK, France and Germany for example is more complex and monitoring is currently carried out at a regional level.
However, in UK a monitoring scheme has recently been proposed to operate at a national level (Chilton and Milne
1994). This is a hierarchical scheme in which a sampling network of three levels and scales is differentially targeted at a number of defined objectives. In the USA trends in groundwater quality are monitored by a series of 54 catchments upon which routine measurements are conducted at regular intervals but with intensive studies rotated between catchments on an approximate lo-year cycle (Alley and Cohen 1991).
Much of the current activity worldwide is devoted to the measurement of thresholds for a wide range of substances for public supply that must not be exceeded for statutory reasons. These measurements may not always be suitable for aquifer monitoring and for use as indicator parameters.
In addition to analytical accuracy and precision, attention needs to be paid to problems of sampling and representativeness. Groundwater sampling must take into account the aquifer framework and heterogeneity, pumping and flow regimes, geographical constraints in the distribution of existing boreholes, the particulate/solute composition and the need to filter, the need for field
Figure 6. The significance of the unsaturated zone as an indicatorfor recharge estimation, record of recharge history, investigation and understanding of geochemical processes and the transport of contaminants.
76 Proceedings
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the UNESCO / NKK IACSAD Workshops on “Wadi Hydrology” and ‘%roundwater Protection ”Table 3. Geoindicators in the groundwater environment.
Priority 1 Issue/Problem t Water $CO3/ DO
+ t Total groundwater reserves
Ix I I
1 PRIMARY INDICATORS r
-1
I *---. I
pH DOC NO3 Cl SO4*
t
Changing water tableI” I
I I I t
Spring dischargeIndices of water quality: storage changes
I I
X X
I 1
At, Ca
* Groundwater salinity
I xI t
I
X X
t I t I I
X X X
I I
3H, %I, “10
* f Acid neutralisation
1 1 Mg/CI, Br, l8O, 2H (TDS,SEC) 1
* f Agricultural impact
I
Urban industrial impactI I
8, PO+ solvents, metals etc (CFC) I1 Radioactive contamination 1
I
Laod use/forestry changeAquifer redox status X X Eh, Fe2+, HS
X X
Depletion of palxowater X 1gO 2” 14c . 3
Chaoging recharge
I x I bO,2” t
t and climatic influence t
I
Mining impactI X I 1 I X I metals I
Saturated zoqe
X
X
X
X
X
X
X
X
X
X
unsaturated zone
X
X
Frequency of measurement&v)
0.25
0.5
77 Proceedirrgr of‘the UNESCO / NWRC f ACSAD W or I o k h p I OTL “Wudi Hydrology “and “Grourtdwater Protection ”
samples ( Foster and Gomes 1989). It should be noted that pumped samples usually represent mixtures from different depths. For monitoring purposes, the origins need to be constrained by supporting data such as pumping rate measurements and geophysical logging.
8 Conclusions
The purpose of indicators is to measure rather precisely those key parameters that can best describe long term trends and effects on the environment. Thus, well constrained sampling, analytical precision and sensitivity become important.
Measurement of relatively few, well-chosen parameters on well-selected and characterised sites becomes more important than large numbers of ‘threshold’ analyses. The health of the aquifer is equally as important in the present context as the information directly related to public health.
The primary and secondary indicators proposed above are applicable to semi-arid regions, serving as proxy data for one or more processes or water quality problems. In semi- arid regions recharge becomes an important control on vulnerability, so that water level and salinity (Cl) monitoring are of high priorities. Human activity poses a threat to quality indirectly by the risk of over-exploitation of resources and directly by contamination. In semi-arid countries the problem of contamination is alleviated somewhat by the slower rates of recharge, but may be exacerbated by extreme hydrological events. Use of indicators and monitoring strategies need to take these factors into account.
References
Alley, WM. & Cohen,P 1991. A scientifically based nationwide assessment of groundwater quality in the United States. Environ. Geol. and Water Sci, 17:17-22.
Allison,G.B., Gee,G. W& Ty1el;S.W 1994. Vadose-zone techniques for estimating groundwater recharge in arid and semi-arid regions. Soil Sci.Soc.Am.J.58:6-14.
Appelo, CA. J & Postma,D. 1993. Geochemistry, groundwater and pollution. Rotterdam: Balkema.
Berger A. R. & lams, W J. 1996. Geoindicators. Assessing rapid environmental changes in earth systems. Balkema, Rotterdam.
Busenberg,E & P1ummel;L.N. 1992. Use of
chlorofluorocarbons (CCUF and CC12F2) as hydrologic tracers and age-dating tools: The alluvium and terrace system of central Oklahoma. Water Resour Res. 28:
22.57-2283.
CEC. 1993. Research and technological development for the supply and use offresh water resources. Report on monitoring and modelling. EUR-14725-EN. Luxembourg:
CEC.
Champ, D. R, Gulens, J & Jackson, R. E. 1979. Oxidation- reduction sequences in groundwaterflow systems.
Canadian J .Earth Sci. 16: 12-23
Chilton PJ and Milne C J. 1994. Groundwater quality assessment: a national strategy for the NRA. Report to National Rivers Authority. British Geological Survey Report WD/94/4OC
Custodio, E. 1992. Hydrological and hydrochemical aspects of aquifer overexploitation. In: Selected papers on aquifer overexploitation. International Association of Hydrogeologists Vo1.3, 3-27. Hanover:Heise,
Darling, WG., Edmunds, W? M., Kinniburgh, D. G. &
Kotoub,S. 1987. Sources of recharge to the basal Nubian sandstone aquifer Butana region, Sudan. In Isotope Techniques in Water Resources Development:205- 224. Vienna:IAEA.
Edmunds, WM. 1996. Indicators in the groundwater environment of rapidenvironmental change. pp 135-150 In A.R Berger & WJ.Iams (eds), Geoindicators. Balkema, Rotterdam.
Edmunds, WM.,Darling, WG. & Kinniburgh, D.G. 1988.
Solute profile techniques for recharge estimation in semi- arid and arid terrain. In I.Simmers (ed) Estimation of Natural Groundwater Recharge:I39-
157.Amsterdam:Reidel.
Edmunds, WM.,Darling, W.G. & Kinniburgh, D.G., Kotoub, S. & Mahgoub, S. 1992. Sources of recharge at Abu Delaig, Sudan. J Hydrol. 131, I-24.
Edmunds, WM.& Wa1tonN.R.G. 1980. A geochemical and isotopic approach to recharge evaluation in semi- arid zones - past and present. In Application of Isotopic techniques in Arid Zone Hydrology. Proc. Advisor Group Meeting, Vienna, I978 :47-68. Vienna: IAEA.
Edmunds, WM.& Gaye, C.B. 1994. Estimating the spatial variability of groundwater recharge in the Sahel using chloride. J.Hydrol.156:47-59.
Edmunds, W M & Smedley, P L.. 1996. Groundwater geochemistry and health: an overview. In Appleton, J.D, Fuge, R and McCall, G.J.H (eds).Geological Society Special Publication No 113pp 91-105.
Edmunds, WM. (in press). Recharge to groundwater in arid and semi-arid regions from the Holocene to the present. Rotterdam. A.A. Balkema
Edmunds,WM.,Gaye, C.B.& Fontes,J-Ch. 1992. A record of climatic and environmental change contained in interstitial waters from the unsaturated zone of northern Senegal. In Isotope Techniques in Water Resources 78 Proceedings
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Development,l991:533-549. Vienna:IAEA
Edmunds, WM. and Wright,E.P 1979. Groundwater recharge and palaeoclimate in the Sirte and Kufra Basins, Libya. J.Hydro1.40:215-245
F0ster;S.S.D and Gomes,D.C. 1989. Ground-water Quality Monitoring: An appraisal of practices and costs. 103~~. Lima:Pan American Centre for Sanitary Engineering and Environmental Sciences (CEPIS) Frapporti, G, Vriend,S. P and van Gaans. 1993.
Hydrogeochemistry of the shallow Dutch Groundwater:
Interpretation of the National Groundwater Quality Monitoring Network. Water Resour Res. 29: 2993-3004.
Gaye, C.B. and Edmunds, WM 1995. Groundwater recharge estimation using chloride, stable isotopes and tritium proJiles in the sands of north-western Senegal.
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Gee,G.W!& Hillel,D. 1988. Groundwater recharge in arid regions:review and critique of estimation methods.
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Hill 0.1984. Relative rates of difSusion of nitrate, chloride, sulphate and water in cracked and untracked chalk. J Soil Sci, 35: 27-34.
Korkman,TE. 1987. Strategies for ground-water monitoring in Denmark. Studies in Environmental Science 17, Quality of Groundwater: 237-246.
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vulnerability in areas of climatic extremes. ~~49-56 In J. Vrba and Zaporozec A (eds) Guidebook on mapping groundwater vulnerability. International Association of Hydrogeologists, Vol16. Heise, Hannover
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Appendix
Geoindicators for the groundwater environment NAME: Groundwater quality
BRIEF DESCRIPTION: The chemistry (quality) of groundwater reflects inputs from the atmosphere, from soil and water-rock reactions (weathering), as well as from pollutant sources such as mining, land clearance, agriculture, acid precipitation, domestic and industrial wastes. The relatively slow movement of water through the ground means that residence times in groundwaters are generally orders of magnitude longer than in surface waters. As in the case of Surface water quality, it is difficult to simplify to a few parameters. However, in the context of geoindicators, a selection has been made of a few important first-order and second-order parameters that can be used in most circumstances to assess significant processes or trends at a time-scale of 50- 100 years.
The following first-order indicators (in italics) of change are proposed, in association with a number of processes and problems, and supported by a number of second order parameters:
1. Salinity: Cl, SEC (specific electrical conductance), S04, Br, TDS (total dissolved solids), Mg/Ca, **O, 2H, F;
2. Acidity and redox status: pH, HCO,, Eh, DO, Fe, As;
3. Radioactivity: 3H, 36C1, 222Rn;
4. Agricultural pollution: NO,, SO,, DOC (dissolved organic carbon), K/Na, P, pesticides and herbicides;
5. Mining pollution: SO,, pH, Fe, As, other metals, F, Sr;
6. Urban pollution: Cl, HCO,, DOC, B, hydrocarbons, organic solvents. During development and use of an aquifer, changes may occur in the natural baseline chemistry that may be beneficial or detrimental to health (e.g. increase in F, As): these should be included in monitoring programs. The quality of shallow groundwater may also be affected by landslides, fires and other surface processes that increase or decrease infiltration or that expose or blanket rock and soil surfaces which interact with downward-moving surface water.
SIGNIFICANCE: Groundwater is almost globally important for human consumption, and changes in quality can have serious consequences. It is also important for the support of habitat and for maintaining the quality of baseflow to rivers. The chemical composition of groundwater is a measure of its suitability as a source of water for human and animal consumption, irrigation, and for industrial and other purposes. It also influences ecosystem health and function, so that it is important to detect change and early warnings of change both in natural systems and resulting from pollution.
1. Salinity: Fresh groundwater may be limited laterally by its interface with sea water and adjacent rock types, or vertically by underlying formation waters. Saline water intrusion into coastal aquifers can result from overpumping of fresh groundwater, or when streamflow decreases (e.g. due to dams or diversions) lead to reduced recharge of aquifers in deltas and alluvial plains. Strong evaporation in areas with shallow water tables may also lead to salinization. Changes in levels of salinity may occur due to natural climate change or due to excessive pumping and irrigation practices that stimulate precipitation of dissolved solids as salts on agricultural lands. It is important to monitor overall changes in salinity using Cl or SEC and, if possible, to characterize the source of the salinity, using one or more secondary indicators.
2. Acidity and redox status: Emissions of SOx and NOx from industrial sources have, in places, led to an order of magnitude decrease in mean rainfall pH. This has accelerated natural weathering rates and reduced the buffering capacity of soils and rocks, causing an increase in acidity of shallow groundwaters especially in areas
deficient in carbonate minerals. Acidification is a major problem to human and ecosystem health in large areas of North America, Northern Europe, Southeast Asia and South America. The impact on surface waters is exacerbated where the buffering effect of HCO, in groundwater baseflow to rivers and lakes is diminished.
Changes in the redox status of groundwater (mainly consequent on the reduction of OJ can also take place rapidly due to microbial or chemical processes in natural systems or as a result of pollution. An increase in acidity (decrease in pH) or a decrease in Eh (redox potential) may give rise to undesirable increases in dissolved metals.
The onset of reducing conditions may, however, have benefits such as in situ de-nitritication.
3. Radioactivity: Natural background radioactivity can be closely related to the presence or absence of rocks and sediments containing uranium or other naturally radioactive materials. Concentrations of dissolved Rn gas provide one means of detecting the presence of natural radioactivity in groundwater [see karst activity]. Of more significance from an environmental point of view is the possible migration of radionuclides to groundwater from thermonuclear testing, nuclear power plants and military installations.
4. Agricultural pollution: Nitrate levels in groundwater have been increasing over recent decades in most countries as a result of drainage of excess fertilizers. Nitrate, and other mobile fertilizer-derived parameters such as K (K/Na), DOC and SO, serve as important tracers of human- induced environmental degradation, though natural denitrification can also occur under reducing conditions (see Acidity). Herbicides and pesticides (insecticides, fungicides) and other agrochemicals may also be mobile in groundwaters and can serve as an index of diffuse pollution beneath agricultural lands over the past 20-30 years. Because analysis is extremely difficult, it is not feasible to use these as indicators. Their presence can, however, be inferred if high concentrations of other indicators are present.
5. Mining pollution: Sulphate derived from the oxidation of sulphide minerals is the best single indicator of pollution from metal and coal mining, oil and gas production, and to a lesser extent from exploration activities. A decrease in pH is generally associated with this process, as are increases in the dissolved loads of Fe and other metals that may contaminate both groundwater and surface waters as acid mine drainage. The problem becomes acute for water supplies and ecosystems as ground- water levels rise following mine closures. F and Sr derived from weathering of associated vein minerals may also serve as secondary indicators.
6. Urban and industrial pollution: The impact of human 80 l’rocredingr
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the UNESCO ! NIVTC IACUD Workshops on “Wadi Hydmiogy ” and “Groundwater Protection ”Indicators of Groundwater Quality
habitation and disposal of wastes characterized by numerous chemicals is invariably evident in the quality of local groundwater. Many chemicals enter the ground, but the deterioration of water quality may be assessed by those constituents that are most mobile. One key issue is to protect deeper, uncontaminated aquifers and to monitor the effects of contaminant plumes moving into surrounding areas. Thus, DOC, Cl and HCO, represent primary indicators of pollution from towns, cities, landfills and waste dumps. Biological impacts may be measured using indicator organisms such as E.coli.
However, harmful microorganisms generally fade out within several hundred meters of flow in groundwater, and an alternative is to measure the breakdown products of these biological processes, such as DOC and HCO,, Secondary indicators include B (where detergents are used), solvents and hydrocarbons.
HUMAN OR NATURAL CAUSE: Both. Changes in natural baseline conditions may occur over the timescales of interest, and may be measured at an individual borehole or spring. Superimposed on these, however, are the greater impacts of the human activities described above.
ENVIRONMENT WHERE APPLICABLE: The main environments of importance from a global viewpoint are those where major aquifers provide water supplies, especially in bottomland settings with saturated riverine or deltaic sediments, generally of limited thickness and high
ENVIRONMENT WHERE APPLICABLE: The main environments of importance from a global viewpoint are those where major aquifers provide water supplies, especially in bottomland settings with saturated riverine or deltaic sediments, generally of limited thickness and high