W.M. Edmunds

Residence time indicators in groundwater : the East Midlands Triassic sandstone aquifer

The East Midlands Triassic (Sherwood Sandstone) aquifer which has been the subject of detailed radiometric age studies, is used to investigate both inert and reactive constituents of groundwater as indicators of residence time. Detailed resampling of the aquifer in 1992 has provided a considerable body of new inorganic geochemistry data, though without radiocarbon. Several inert indicators are defined including the isotopic ratios δ18O, δ2H, 36Cl, noble gas ratios, as well as the halogen elements (Cl, Br, F, I) and their element ratios. These form a group of essentially unreactive tracers primarily reflecting changing rainfall inputs and palaeoclimatic conditions, except at outcrop where human impacts are also seen clearly. The concentrations of Cl, mainly from atmospheric sources, remain below 25 mg l−1 Cl over a distance of some 30 km from outcrop. Reactive indicators, the result of time-dependent water–rock interactions, include δ13C, Mg/Ca, Sr/Ca, Na/Cl and show diagnostic trends along the flow lines. However the concentrations of certain trace elements — Li, Rb, Cs, Mn and Mo — which are not limited by solubility constraints show linear trends along the present day flow gradient. This water–rock interaction is taking place in groundwaters with low total mineralisation and it can be demonstrated that reactions involving these elements and isotopes are occurring entirely within the aquifer since high salinity groundwaters are found below the Sherwood Sandstone. There is good correlation between some of the elements and 14C activities in the 1977 data set and this has been used to derive a concentration–age relationship for the 1992 set of data. A chemical timescale for the aquifer is then established using (a) Li and (b) a combination of five trace elements. The age of the fresh groundwater is thus shown to be up to 100 ka BP, indicating the likelihood of semi-continuous recharge during the Devensian glacial period preceding the glacial maximum, when no recharge occurred (10–20 ka BP). This approach may be of value in extending groundwater dating beyond the radiocarbon timescale as well as interpreting sites where no radiocarbon data are available. The scatter from the mean age line indicates those groundwaters which are derived from either rapid or slower than average flow zones within the aquifer, as well as age stratification. This stratification is borne out by a depth profile of groundwater from a new borehole near to outcrop which is shown, by a combination of chemical and isotopic tracers, to contain Holocene overlying late Pleistocene water.

Use of abstraction regime and knowledge of hydrogeological conditions to control high-fluoride concentration in abstracted groundwater: San Luis Potosí basin, Mexico

Significant amounts of fluoride are found in the abstracted groundwater of San Luis Potosi. This groundwater withdrawal induces a cold, low-fluoride flow as well as deeper thermal fluoride-rich flow in various proportions. Flow mixing takes place depending on the abstraction regime, local hydrogeology, and borehole construction design and operation. Fluoride concentrations (≈3.7 mg l−1) could become higher still, in time and space, if the input of regional fluoride-rich water to the abstraction boreholes is enhanced. It is suggested that by controlling the abstraction well-head water temperature at 28–30 °C, a pumped water mixture with a fluoride content close to the maximum drinking water standard of 1.5 mg l−1 will be produced. Further, new boreholes and those already operating could take advantage of fluoride solubility controls to reduce the F concentration in the abstracted water by considering lithology and borehole construction design in order to regulate groundwater flow conditions.

Baseline geochemical conditions in the Chalk aquifer, Berkshire, U.K.: a basis for groundwater quality management

The Chalk aquifer is the most important British aquifer and is also important over much of northern Europe. Aquifer protection requires a sound knowledge of the baseline conditions and how these might vary, or have varied, with time. This detailed geochemical study of a representative area of Chalk in Berkshire, U.K., includes a consideration of several components: (1) the inputs from the atmosphere; (2) the interstitial water of the soil and the unsaturated zone; (3) the interstitial water in the confined and unconfined sections of the aquifer; and (4) the saturated, mainly fissure flow, along the hydraulic gradient which forms an important water supply of the Thames Valley region. Atmospheric inputs form an important source of some elements, but the dominant chemical characteristics of the Chalk groundwater are acquired during percolation through the soil and the upper unsaturated zone. During saturated flow downgradient the chemistry is modified mainly by incongruent reactions of the carbonate matrix and by redox reactions, and only to a minor extent by exchange reactions and mixing with residual saline connate water. The incongruent reaction of carbonate results in a marked increase in the Mg/Ca ratio and the Sr and 13C contents of the groundwater with increased residence time. Oxygen concentrations are reduced mainly by oxidation of Fe2+, and the onset of reducing conditions allows dissolved Fe2+ to increase and rapid denitrification to occur. The salinity profile through the confined Chalk confirms that residual connate water, up to one-fifth sea water concentration, still remains at depth, and this accounts for some salinity increase in the confined groundwater resulting from fissure water.pore water diffusional exchange. Timescales for groundwater movement have been established using tritium, radiocarbon, and indirectly using inert gas ratios and stable isotope ratios. On balance, it is concluded that all abstracted water is of Holocene age, although inert gas temperatures indicate cooler climatic conditions for recharge for some of the confined groundwater. The implications for development and aquifer protection are discussed, especially the prospect of natural in situ denitrification, problems of Fe solubility, and the recognition of groundwater of different maturities.

Mobility of arsenic in groundwater in the Obuasi gold-mining area of Ghana: some implications for human health

Abstract Arsenic in drinking water from streams, shallow wells and boreholes in the Obuasi gold-mining area of Ghana range between < 2 and 175 μgl−1. The main sources are mine pollution and natural oxidation of sulphide minerals, predominantly arsenopyrite (FeAsS). Streamwaters have been most affected by mining activity and contain some of the highest As concentrations observed. They are also of poor bacteriological quality. Some of the streams have a high As(III) content (As(III)/Astotal > 0.5), probably as a result of methylation and reduction reactions mediated by bacteria and algae. Concentrations of As in groundwaters reach up to 64 μgl−1, being highest in deeper (40–70 m) and more reducing (220–250 mV) waters. The As is thought to build up as a result of the longer residence times undergone by groundwaters in the deeper parts of the aquifer. The proportion of As present as As(III) is also higher in the deeper groundwaters. Deep mine exploration boreholes (70–100 m) have relatively low As contents of 5–17 μgl−1, possibly due to As sorption onto precipitating ferric oxyhydroxides or to localized low As concentrations of sulphide minerals. Median concentrations of inorganic urinary As from sample populations in two villages, one a rural streamwater-drinking community and the other a suburb of Obuasi using groundwater for potable supply, were 42 μgl−1 and 18 μgl−1 respectively. The value for the community drinking groundwater is typical of background concentrations of urinary As. The slightly higher value for the streamwater-drinking community probably reflects different provenance of foodstuffs and higher As concentrations of water sources local to the village. The low value obtained for the inhabitants of the Obuasi suburb, living close to and abstracting groundwater from the area of major mining activity, suggests that groundwater can form a useful potable supply of good inorganic quality provided that deep, long residence time sources are avoided.

Estimating the spatial variability of groundwater recharge in the Sahel using chloride

Thirteen interstitial water chloride profiles have been obtained from the unsaturated zone of the coastal Quaternary aquifer in Senegal over depth intervals ranging between 7.0 and 35.5 m; three of these profiles reached the water table. A regional study of 119 dug wells was also carried out over an area of some 1600 km2 for the measurement of chloride, as well as other parameters, in samples taken from the water table. These two sets of data have been used, together with rainfall input chemistry collected over a 3 year interval to derive regional estimates of recharge for this area typical of the Sahel margin (mean annual rainfall 1970–1990 around 280 mm). Recharge estimates range from 13 000 to 1100 m3 km−2 year−1. It is concluded that the conjunctive use of interstitial water profiles and regional shallow groundwater chemistry provide an inexpensive technique for recharge estimation which can be widely applied in sedimentary terrains.

Solute Profile Techniques for Recharge Estimation in Semi-Arid and Arid Terrain

Conventional methods for recharge estimation have limitations when applied to arid and semi-arid regions; the use of tritium profiles is also not always applicable. Unsaturated zone solute profiles, using a reference solute such as chloride, offer an alternative technique. Sampling may be undertaken by percussion drilling, augering or from dug wells; the methods developed are described and examples discussed. Recharge estimates using chloride profiles from Cyprus (420 mm mean annual rainfall) are in good agreement with results estimated from tritium profiles and indicate a mean annual recharge of around 50 mm/year. In Central Sudan (180 mm mean annual rainfall), good agreement was found between adjacent unsaturated zone chloride profiles and these indicated a net annual direct recharge via interfluve areas of around 1 mm/year. It is concluded that solute profiles offer a cheap and effective tool for estimating direct recharge in porous lithologies of semi-arid regions and also for investigating recharge history, providing input data for chloride are available. In more arid regions, however, a component of discharge may occur during hyperarid episodes. Further validation of moisture composition using stable isotope techniques is required under such conditions.

Natural groundwater quality

Foreword. 1. Groundwater Baseline Quality. W.M. Edmunds and P. Shand. 2. The Baseline Inorganic Chemistry of European Groundwaters. P. Shand and W. M. Edmunds. 3. Organic Quality of Groundwaters. D.C. Gooddy and K. Hinsby . 4. Geochemical Modelling of Processes Controlling Baseline Compositions of Groundwater. D. Postma, C. Kjoller, M. Sogaard Andersen, M.T. Condesso de Melo and I. Gauss . 5. Part 1: Timescales and Tracers. R. Purtschert . 5. Part 2: Dating Examples in European Reference Aquifers. R. Purtschert, J. Corcho-Alvarado and H.H. Loosli . 6. Identifying and Interpreting Baseline Trends. M. Van Camp and K. Walraevens . 7. Monitoring and Characterisation of Natural Groundwater Quality. M.T. Condesso de Melo, E. Custodio, W.M. Edmunds and H. Loosli . 8. Natural Groundwater Quality: Policy Considerations and European Opinion. E. Custodio, P. Nieto and M. Manzano . 9. The Chalk Aquifer of Dorset, UK. W.M. Edmunds and P. Shand . 10. Groundwater Baseline Composition and Geochemical Controls in the Donana Aquifer System (SW Spain). M. Manzano, E. Custodio, M. Iglesias and E. Lozano . 11. The Aveiro Quaternary and Cretaceous Aquifers, Portugal. M.T. Condesso de Melo and M.A. Marques da Silva. 12. The Neogene Aquifer, Flanders, Belgium. M. Coetsiers and K. Walraevens . 13. The Miocene Aquifer of Valreas, France. F. Huneau and Y. Travi . 14. The Miocene Sand Aquifers, Jutland, Denmark. K. Hinsby and E. Skovbjerg Rasmussen . 15. Tracer Based Study of the Badenian Bogucice Sands Aquifer, Poland. S. Witczak, A. Zuber, E. Kmiecik, J. Kania, J. Szczepanska and K. Rozanski . 16. The Cambrian-Vendian Aquifer, Estonia. R. Vaikmae, E. Kaup, A. Marandi, V. Raidla and L. Vallner . 17. The Cenomanian and Turonian Aquifers of the Bohemian Cretaceous Basin, Czech Republic. T. Paces, J.A. Corcho Alvarado, Z. Herrmann, V. Kodes, J. Muzak , J. Novak, R. Purtschert, D. Remenarova and J. Valecka . 18. Quality Status of the Upper Thracian Plio-Quaternary Aquifer, South Bulgaria. M. Machkova, B. Velikov, D. Dimitrov, N. Neykov and P. Neytchev . 19. The Mean Sea Level Aquifer - Malta and Gozo. J. Mangion and M. Sapiano . 20. The Natural Inorganic Chemical Quality of Crystalline Bedrock Groundwaters of Norway. B. Frengstad and D. Banks . 21. Natural Groundwater Quality - Summary and Significance for Water Resources Management. W.M. Edmunds and P. Shand . Index

Bromine geochemistry of British groundwaters

Abstract The concentrations of Br in potable groundwaters in the United Kingdom range from 60 to 340 μg 1--1. The occurrence of Br is described in terms of the Br/Cl weight ratio which enables small changes in bromide concentt|ations to be assessed in terms of salinity. Median values of Br/Cl in groundwaters range from 2.60 to 5.15 × 10-3 compared with a sea water ratio of 3.47 × 10-3. In recent shallow groundwaters the Br/Cl ratio is rather variable in response to a range of natural and anthropogenic inputs (marine and industrial aerosols, industrial and agricultural chemicals including road salt). Some slight enrichment in Br/Cl also occurs naturally during infiltration as a result of biogeochemical processes. Evolution of Br/Cl along groundwater flow lines reflects the sources of increasing salinity; either the influence of marine sedimentary formations or evaporites. The groundwaters in the Triassic sandstones of the English Midlands show significant Br depletion due to the evaporite source, in contrast to groundwaters in Cumbria. Br/Cl ratios in the Sherwood Sandstone of the East Midlands mainly reflect the natural input sources and can be used to help understand the palaeohydrology.

Isotopic evidence for palaeowaters in the British Isles

Before the 20th century, groundwater circulation in the aquifers of the British Isles had largely adjusted to the temperate maritime climate and sea levels established over the past 10 ka since the end of the Pleistocene. However, in the last 100 a this natural regime has been disturbed by abstraction of water for public supply and industrial use, and palaeowaters from earlier recharge episodes are now becoming a factor to be considered in water balance estimates. This paper presents a synthesis of the existing palaeowater distribution in the British Isles, based on isotopic evidence (δ18O, δ2H and and14C). As such, it has relevance to palaeoclimatic studies in addition to the water resource implications. The Triassic basins of England and Northern Ireland contain saline waters beyond the range14C dating (>; 40 ka). Stable isotopic ratios show enrichment in some basins and depletion in others, without an overall pattern that would explain all the observed compositions. The results for the Wessex basin suggest recharge in pre-Quaternary times, but for the other basins some flushing by Pleistocene or Holocene meteoric waters is indicated. Isolated occurrences of apparently long-residence waters are found elsewhere throughout the British Isles, for example from Carboniferous and Lower Palaeozoic strata. In such cases, environmental isotopes are more useful as constraints on hydrogeological models than as indicators of palaeoconditions. Major water supply aquifers are restricted almost entirely to England. The two sandstone formations (Triassic Sherwood Sandstone and Cretaceous Lower Greensand) have a greater range of stable isotopic values between phreatic and confined conditions than the two carbonate formations (Jurassic Lincolnshire Limestone and Cretaceous Chalk). This indicates that the sandstone aquifers are better archives of information on palaeoconditions than the carbonate aquifers. They show that atmospheric circulation patterns over Britain have probably remained the same since the late Pleistocene. However,14C data from all four of the major aquifers emphasise the hiatus in recharge during periglacial conditions which occurred between the late Pleistocene and early Holocene.

Lakes, groundwater and palaeohydrology in the Sahel of NE Nigeria: evidence from hydrogeochemistry

The geochemistry of natural waters from rain, soil, the unsaturated zone, shallow aquifers, lakes and the stratified aquifer system of the Chad Basin sediments has been investigated to interpret modern hydrological processes and to reconstruct the palaeohydrology of NE Nigeria, a type region of the southern Sahel. Recharge to the confined Middle and Lower aquifers, recorded in NE Nigeria, occurred between 24 and 18.6 ka BP, prior to the last glacial maximum. The mean annual temperature at this time derived from dissolved noble gas ratios was at least 6°C cooler than at the present day. This groundwater is not coupled to the active modern recharge cycle and was not reactivated during the Holocene wet phases as elsewhere in the Saharan region, a reflection of changing lake levels and/or of changing climatic regimes. The absence of groundwater recharge at the time of the last glacial maximum supports other evidence for aridity at this time. Present day direct recharge rates in the Manga Grasslands are high (mean 44 mm a −1). However, regional recharge in NE Nigeria at the present day is even higher (60 mm a −1) emphasizing the importance of infiltration from surface runoff as input to groundwater. The present study confirms that the shallow aquifer in the region contains significant renewable groundwater resources. However the confined aquifer of the Chad Basin clearly contains palaeowater and this declining artesian basin will need careful conservation prior to a return to traditional water use methods via improved management of the renewable waters in the shallow phreatic aquifers.