W.G. Darling

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.

A stable isotope study of recharge processes in the English Chalk

The δ2H and δ18O values of water contained in unsaturated Chalk have been measured in drillcore profiles taken over a period of 28 months on arable land in Cambridgeshire, eastern England. The isotopic composition of rainfall and lysimeter drainage on the same site has also been monitored over a period of four years. The distribution of isotope values in the upper few metres of soil and unsaturated Chalk shows that the mechanism of infiltration is not simple piston-like downward displacement since the scale of vertical fluctuations is incompatible with estimated annual infiltration and the apparent rates of movement of matrix water derived from thermonuclear tritium studies. Lysimeter drainage is not isotopically identical to matrix water at the same depth suggesting that different routes to water table are associated with consistently different isotopic content. Although there is an apparent difference between matrix water beneath arable and permanent grassland, these differences appear to cancel out in the composition of bulk recharge beneath these land types. The regional aquifer has a uniform isotopic composition similar in value to weighted average rainfall and lysimeter drainage, suggesting that the recharge-abstraction system has been in isotopic equilibrium for many years despite changes in pumping rates and land use. Two other Chalk sites in southern England have been investigated in less detail but indicate clear difference in infiltration behaviour from that seen in Cambridgeshire; this seems to be related to a higher matrix conductivity and annual rainfall amount.

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.

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.

ISOTOPE STUDIES OF PIPEFLOW AT PLYNLIMON, WALES, UK

Residence times and flow paths of pipe and stream flow were studied during low flow in the Nant Gerig and Gwy experimental catchments at Plynlimon in mid-Wales, UK, using a two-month time series of natural deuterium and electrical conductivity data from perennial and ephemeral pipe flow, stream flow, groundwater and rainfall. Low flow in both the perennial pipe and the stream was maintained by ‘old’ groundwater discharge. This groundwater was at least 40 days old. Flow in the ephemeral pipe was dominated by old groundwater and was only slightly affected by direct inputs of new water. Although direct rainfall inputs contributed minimally to runoff in the perennial pipe and the stream, rainfall influenced the isotopic and chemical character of the groundwater. Rainfall also affected the water-table elevation, which determined the flashiness of the perennial pipe flow and whether the ephemeral pipe flowed. The isotope and electrical conductivity data suggest that storm runoff in both the main pipe and the stream is overwhelmingly old water. A sensitivity analysis suggests that the old water is supplied both from near-stream groundwater and upslope groundwater delivered by the ephemeral pipes.

The origin of hydrothermal and other gases in the Kenya Rift Valley

The Kenya Rift Valley (KRV) is part of a major continental rift system from which much outgassing is presently occurring. Previous research on gases in the KRV has tended to concentrate on their geothermal implications; the present paper is an attempt to broaden the interpretation by consideration of new data including helium and carbon isotope analyses from a wide cross-section of sites. In order to do this, gases have been divided into categories dependent on origin. N2 and noble gases are for the most part atmospherically derived, although their relative concentrations may be altered from ASW ratios by various physical processes. Reduced carbon (CH4 and homologues) appears to be exclusively derived from the shallow crust, with thermogenic δ13C values averaging −25‰ PDB for CH4. H2 is likely also to be crustally formed. CO2, generally a dominant constituent, has a narrow δ13C range averaging −3.7‰ PDB, and is likely to be derived with little modification from the upper mantle. Consideration of the ratio C/3He supports this view in most cases. Sulphur probably also originates there. Ratios of 3He/4He reach a MORB-like maximum of 8.0 R/RA and provide the best indication of an upper mantle source of gases beneath the KRV. A correlation between 3He/4He and the hydrocarbon parameter log (C1/ΣC2–4) appears to be primarily temperature related. The highest 3He/4He ratios in spring waters are associated with basalts, perhaps because of the leaching of basalt glasses. There may be a structural control on 3He/4He ratios in the KRV as a whole.

Hydrothermal hydrocarbon gases. 1, genesis and geothermometry

Various sources for hydrothermal CH4 have been proposed over the years. While C isotope studies have narrowed the possibilities, enough higher hydrocarbon gas data now exist both to supplement the isotopic data and to permit additional deductions regarding origins. Comparison of typical C1–C6 data for gases of various origins (from sedimentary and crystalline rocks, and hydrothermal systems) reveals certain characteristics. Apart from isotopic differences, hydrothermal hydrocarbons differ from sedimentary hydrocarbons mainly in possessing tendencies towards a relative excess of CH4, higher normal/iso ratios for butane and pentane, and relatively high amounts of C6 gases. Despite these differences, consideration of the evidence indicates that hydrothermal hydrocarbon gases in most cases originate like sedimentary basin gases by thermal degradation of organic matter in the relatively shallow subsurface. The principal characteristic of these hydrothermal gases, “excess” CH4, appears to have a geothermometric function. The following empirical relationship has been derived: t°C=57.8 log(CH4/C2H6)+96.8, which fits moderately well a range of geothermal fields worldwide. This gas geothermometer may be particularly applicable during geothermal exploration in areas where there is little direct knowledge of subsurface conditions.

Stable isotopic aspects of fluid flow in the Krafla, Námafjall and Theistareykir geothermal systems of northeast Iceland

Abstract Water and steam condensates from the Krafla, Namafjall and Theistareykir geothermal systems have been studied using hydrogen and oxygen stable isotope ratios to gain more information on thermal fluid flow, the effect of rifting on individual fields and the provenance of recharge to these fields. At Krafla there are minor differences between individual wellfields, but recharge is essentially local. Only one well shows signs of a change in composition over a 6-year production period, but a local volcanic eruption in 1984 affected fumarole steam compositions. The nearby Namafjall field has isotopic compositions suggesting that recharge is ultimately derived from a long distance to the south, perhaps by a combination of surface and groundwater flow. Some local spring and shallow well compositions have been affected by the post-1975 rifting episode. The undrilled Theistareykir field has an inflow estimated to be of similar isotopic composition to Namafjall. Some fumaroles have an anomalously depleted δ18O composition, and Rayleigh condensation is postulated to explain them.

Investigation into the use of deuterium as a tracer for measuring transpiration from eucalypts

A study into the use of deuterium oxide as a tracer for measuring transpiration rates from trees was carried out on three year old Eucalyptus teretecornis trees growing in a plantation in southern India. Two trees, with diameters equal to that of the mean of the plantation, were injected at their bases with 3.0 and 1.5 ml of D2O, respectively. Four samples of transpired water were collected per tree as condensate in plastic bags which enclosed branches in the top and bottom of the canopy. The coefficient of variation of the integral of the concentration-time curves (∫Cdt) from the four sampling positions on each tree was 11 and 15%. Unlike another reported study ∫Cdt was found to be larger (on average by 25 ± 7%) from samples taken at the top of the canopy compared with those at the bottom. Transpiration rates were calculated as 0.86 ± 0.05 and 0.85 ± 0.07 mm per day from the two trees. The internal consistency of the results suggest that the method is well suited to transpiration studies from young eucalypts; on safety and economic considerations deuterium is to be preferred to tritium as a tracer.

Sources of recharge at Abu Delaig, Sudan

Abstract Various components of the water cycle in and around Abu Delaig, a small town in a semiarid region of Sudan, have been investigated with geochemical and isotopic techniques to determine the sources of groundwater recharge. Rain samples (total deposition during storms) contain significant concentrations of dissolved solids for a continental site (mean 4.6 mgl −1 for 1982–1985). The heaviest rains important for recharge tend to have the lowest chlorinities, and are also the most negative isotopically. Wadi floods generally have lower chlorinity than the rainfall indicating the proportionally lower amounts of dust in the more intense rainfall events. Shallow ground waters at Abu Delaig have relatively evolved compositions (higher Mg/Ca ratios) compared with rainfall and wadi floods, yet they also contain tritium and retain isotopic signatures similar to the local rainfall. They also have a distinctive chemistry compared with the deeper ground waters in the region and recharge from the former to the latter is considered to be insignificant. Ground waters in the unsaturated zone are saline and have isotopic compositions highly enriched in heavy isotopes compared with rainfall, indicating strong evaporation. The chloride balance indicates that mean recharge rates are 0.2–1.3 mm year −1 through the interfluve areas. It is concluded that the only significant replenishable resources at Abu Delaig are from wadi recharge during floods and that direct regional recharge is insignificant. Fluctuations in the water table, however, lead to solutes from the lower unsaturated zone contributing to the chemistry of the shallow ground waters. Elsewhere it is possible that wadi recharge may be a possible route for deeper replenishment, but this needs to be demonstrated by dedicated experiments.