John H. Tellam

Marker species for identifying urban groundwater recharge sources: A review and case study in Nottingham, UK

Abstract Urban environments significantly alter the nature of recharge to underlying aquifers. Direct precipitation is reduced, but additional recharge may result from storm water runoff, mains supply leakage and sewer leakage. If urban aquifers are to be effectively and sustainably managed, it is vital that these recharge sources should be identified and quantified. A sound theoretical approach is the use of marker species for identifying the three principal sources of urban recharge (precipitation, mains and sewers). The ideal marker species should be unique to a particular recharge source (irrespective of geographic location), and easily identifiable in the groundwater system, enabling quantification of that source. A review of potential markers and a detailed study of the aquifer beneath the city of Nottingham, UK, was unable to find suitable markers for precipitation and mains leakage. Trihalomethanes, which are chlorination by-products, and so a potential marker of mains water, were hardly detected in either mains or groundwater. More potential markers are available for sewage, including d-limonene, which is a new ingredient in some detergents. For shallow groundwater, the most effective means of identifying sewage recharge was a combination of stable nitrogen isotopes and microbiological indicators; effectively a sewage “fingerprint”. This study confirms the need for a multi-component approach rather than using individual marker species. Additionally it demonstrates that the impact of sewer leakage on groundwater quality beneath Nottingham is generally not high.

Migration and attenuation of agrochemical pollutants: insights from isotopic analysis of groundwater sulphate

Abstract Existing hydrochemical and hydrogeological models of pollution migration and attenuation in the Lincolnshire Limestone aquifer of eastern England have been examined in the light of the results of a groundwater sulphate sulphur isotope investigation. This has allowed the distinction of different sources of sulphate and their relative importance in different parts of the aquifer. The principal sources are 34 S-depleted inputs, derived from the oxidation of pyrite within both the aquifer matrix material and the overlying mudstone deposits, and 34 S-enriched anthropogenic inputs which are derived from acid rain fallout augmented by agrochemicals. Groundwaters sampled over the outcrop zone of the aquifer have sulphate δ 34 S dominated by contemporary acid rain inputs in the recharge waters. A down-dip decrease in the 34 S content of groundwater sulphate over the unconfined and shallow confined areas of the aquifer is indicative of a progressive increase in the significance of pyrite-derived sulphate in the system. The contribution of sulphate from this source is large and indicates that pollution front penetration (previously based on total sulphate concentrations) is more restricted than previously thought. Moreover, the extent of pyrite oxidation is greater than can be accounted for by dissolved O 2 and the additional component corresponds to that which would be expected from reduction of nitrate using pyrite as the electron donor. We suggest that this mechanism is responsible for denitrification in the aquifer, but that it will be ultimately limited by pyrite availability near fissure surfaces where the reaction takes place.

Source, type and extent of inorganic contamination within the Birmingham urban aquifer system, UK

A survey of the inorganic quality of the groundwater of the Birmingham Triassic sandstone aquifer was carried out by sampling seventy 80–150 m deep industrial abstraction boreholes, 11 shallow monitoring piezometers, two submerged tunnels and two flooded industrial basements, and a set of trial pits. Borehole logging and historical data were also obtained. Given that the city of Birmingham is as long established as most industrial conurbations in the world, the degree of groundwater inorganic contamination as indicated by abstraction well samples is not excessive. Out of the 28 determinands measured, only NO−3 and Ba2+ consistently approach and exceed their European Community Maximum Admissible Concentrations standards for drinking water and the barium is being supplied by the aquifer rather than having an anthropogenic source. However, a few abstraction boreholes have highly contaminated groundwaters, with for example milligram per litre levels of heavy metals. In addition, there is a correlation between land use and chemistry, with the highest salinity, sulphate, chloride, sodium, boron, and total heavy metal concentrations being associated with metal-working sites. High concentrations of heavy metals (and NH+4 are unexpected in the near-neutral pH Triassic sandstone groundwaters given the high sorption capacity of the sandstone: it is concluded that one reason for the occasionally high concentrations observed is the presence of very locialized high concentration recharge. Data from shallow piezometers confirm the considerable degree of heterogeneity in groundwater quality on a small scale — the pollution is mainly from point sources. Sampling from piezometers, tunnels, and basements demonstrates that the groundwater concentrations at shallow depths are often much higher than the pumped water analyses suggest. However, nitrate groundwaters appear to vary much less with depth than other species, reflecting a longer history of supply and a wider range of sources. Geophysical logging and sampling of well columns, and other observations indicate that the intermittent pumping of abstraction wells results in the pumped water quality being biased towards the shallower, high concentrations groundwaters rather than being a uniform mixture from all screened depths. Historical data show various trends, and these can be related to local hydrogeology, land use, and abstraction history. Comparisons of organic and inorganic pollution show that distributions are very different, reflecting differences in physical transport processes, chemical interactions, and histories of chemical usage.

Hydrochemistry of the saline groundwaters of the lower Mersey Basin Permo-Triassic sandstone aquifer, UK

Saline groundwater occurs at depth in the Triassic Sandstones of the northern end of the Cheshire Basin in northwest England. Major and minor ion data suggest that the origin of the salinity is dissolution of evaporite deposits; the salinity probably developed in an interval following the Tertiary inversion of the Basin which would have allowed faulted contact between the sandstones and the overlying evaporite sequence of the Mercia Mudstone Group. Isotope evidence suggests the saline waters to have been considerably diluted by presumably dispersive mixing with a meteoric water recharged under cool, and therefore Quaternary, climatic conditions. Comparison with other UK Permo-Triassic Basin brines suggests the Merseyside waters are not unusual, except in terms of the importance of exchange reactions and dilution by Quaternary recharge waters. These factors are due to the shallow maximum burial depth of the aquifer and the preservation of smectites, and also to the geometry of the aquifer and its low-permeability faults. In the light of the water chemistry data it is suggested, in a slight modification of the proposal of Naylor et al. Q. Geol. Soc. Lond., 146: 685–699, 1988), that the Cheshire Basin mineralization resulted from mixing of the relatively oxidized, SO4-rich evaporite brines with the reduced, SO4-poor, metal-rich, Barich, Coal Measures brines which were expelled through faults from the Coal Measures during Basin inversion: some degree of oxyhydroxide-stripping of the sandstones by the Coal Measures brines would have enhanced metal concentrations. The water resulting from the mixing of brines would have been far from equilibrium, and the barite and sulphide precipitation followed along the faults.

Quantification of the water balance and hydrogeological processes in the vicinity of a small groundwater-fed wetland, East Anglia, UK

Abstract Badley Moor Fen exists in a region of the UK where potential evapotranspiration exceeds annual rainfall. The wetland is maintained by an input of Chalk groundwater in an area where the groundwater aquifer is regionally confined below boulder clay. The large groundwater contribution to the water balance arises from a unique local geological situation on the margin of the valley floor that allows the vertical movement of groundwater to the surface under a strong upward head gradient. The groundwater contribution is the dominant water input and maintains saturation within the immediate vicinity of the source of groundwater. Surface water outflow, via adjacent drainage channels, is the major output. The groundwater component is also important in that, as a result of hydrochemical change in the groundwaters with flow towards the surface, tufa has been precipitated. The calcium bicarbonate rich groundwater and tufa accumulation have given the site a unique character, with an exceptionally rich calcareous fen community.

Controls on bacterial sulphate reduction in a dual porosity aquifer system: the Lincolnshire Limestone aquifer, England

Abstract Chemical and sulphur isotopic analyses are presented of fissure-waters and pore-waters in the deep confined zone of a dual porosity carbonate aquifer. Some of the fissure-waters show good evidence for bacterial sulphate reduction, with low concentrations of sulphide present which is strongly to moderately depleted in 34 S relative to sulphate. The sulphur geochemistry is best interpreted as mixing between: (i) a reduced water with sulphide ∼60‰ depleted in 34 S relative to sulphate; and (ii) a sulphate-rich water from up-dip in the aquifer. In addition, sulphide oxidation occurs where sufficiently oxidizing water is drawn in by abstractions. The large isotope fractionation factor associated with the sulphidic waters is probably the result of redox cycling of sulphur with little net reduction, rather than a true kinetic fractionation factor. By contrast, pore-waters in the “sulphate reducing zone” show little or no evidence for the effects of sulphate reduction, despite the fact that the pore-waters represent a significant reservoir of sulphate for reduction. Some pore-waters have been modified recognizably by diffusional exchange with the fissure-waters, but the aquifer matrix has not been colonized by sulphate reducing bacteria, probably because porethroats in the limestone are too small for bacteria to pass. Physical exclusion of bacteria from the aquifer matrix and limited diffusional exchange are likely to exert fundamental controls on bacterial redox processes in dual porosity aquifer systems and other systems with low permeability due to small pore interconnections.

The value of iodide as a parameter in the chemical characterisation of groundwaters

Abstract Brackish and saline groundwaters can severely constrain the use of fresh groundwaters. Their chemical characterisation is important in understanding the hydraulic conditions controlling their presence in an aquifer. Major ions are frequently of limited value but minor ions can be used. Iodide in groundwater is particularly significant in many environments due to the presence of soluble iodine in aquifer matrix materials. Iodide is found in groundwaters in parts of the English Chalk aquifer in concentrations higher than are present in modern seawater. Its presence is considered as a indication of groundwater residence and is of use in the characterisation of fresh as well as saline waters. Under certain circumstances modern seawater intrusion into aquifers along English estuaries produces groundwaters which are easily identified due to iodide enrichment from estuarine muds. In other environments iodide concentrations are of value in distinguishing between groundwaters in limestones and shaly gypsiferous rocks as shown by a study in Qatar, while in an alluvial aquifer study in Peru iodide has been used to identify groundwaters entering the aquifer from adjacent granodiorites.

The morphology of a saline groundwater body: Its investigation, description and possible explanation

In many countries knowledge concerning the occurrence and behaviour of saline groundwaters is becoming even more vital as water resource and waste disposal projects become more sophisticated. This paper outlines the approach adopted in, and the result obtained from, a study of saline water occurrence in an important United Kingdom aquifer. Saline groundwater, of concentration up to at least 100 g l−1 Cl−, occurs as a layer of variable thickness below fresh groundwater in the Permo-Triassic sandstones of the Lower Mersey Basin of northwestern England. As part of a general water resources study, a detailed investigation of the morphology of the saline water body was undertaken using modern and historical chemical data, surface geophysics, borehole geophysics, open hole depth sampling, and borehole core porewater analysis. It was found that Cl− concentrations at all sites show a pattern of steady increase to depths of at least 320 m. The surface of the saline water body has a considerable topographic variation. Saline water occurs at highest elevations below the roughly east—west trending Mersey Valley where it approaches ground level, but declines in elevation both to the north and south: In the north the saline “interface” meets the rising base of the sandstone sequence at a depth of around 280 m. Evidence from chemical and viscous flow analogue studies suggests that this ridged topography can be explained in terms of flushing and regional upconing in response to low groundwater heads in the Mersey Valley.

Attenuation of landfill leachate by UK Triassic sandstone aquifer materials: 1. Fate of inorganic pollutants in laboratory columns

Abstract The attenuation of inorganic contaminants in acetogenic and methanogenic landfill leachate by calcareous and carbonate-deficient, oxide-rich Triassic sandstone aquifer materials from the English Midlands was examined in laboratory columns. Aqueous equilibrium speciation modelling, simple transport modelling and chemical mass balance approaches are used to evaluate the key processes and aquifer geochemical properties controlling contaminant fate. The results indicate that leachate–rock interactions are dominated by ion-exchange processes, acid–base and redox reactions and sorption/precipitation of metal species. Leachate NH 4 is attenuated by cation exchange with the aquifer sediments; however, NH 4 migration could be described with a simple model using retardation factors. Organic acids in the acetogenic leachate buffered the system pH at low levels during flushing of the calcareous aquifer material. In contrast, equilibrium with Al oxyhydroxide phases initially buffered pH (∼4.5) during flushing of the carbonate-deficient sandstone with methanogenic leachate. This led to the mobilisation of sorbed and oxide-bound heavy metals from the aquifer sediment which migrated as a concentrated pulse at the leachate front. Abiotic reductive dissolution of Mn oxyhydroxides on each aquifer material by leachate Fe 2+ maintains high concentrations of dissolved Mn and buffers the leachate inorganic redox system. This feature is analogous to the Mn-reducing zones found in leachate plumes and in the experiments provides a sink for the leachate Fe load and other heavy metals. The availability of reactive solid phase Mn oxyhydroxides limits the duration of redox buffering and Fe attenuation by these aquifer sediments. Aquifer pH and redox buffering capacity exert a fundamental influence on leachate inorganic contaminant fate in these systems. The implications for the assessment of aquifer vulnerability at landfills are discussed and simple measurements of aquifer properties which may improve the prediction of contaminant attenuation are outlined.

Attenuation of landfill leachate by UK Triassic sandstone aquifer materials 2. Sorption and degradation of organic pollutants in laboratory columns

The sorption and degradation of dissolved organic matter (DOM) and 13 organic micropollutants (BTEX, aromatic hydrocarbons, chloro-aromatic and -aliphatic compounds, and pesticides) in acetogenic and methanogenic landfill leachate was studied in laboratory columns containing Triassic sandstone aquifer materials from the English Midlands. Solute sorption and degradation relationships were evaluated using a simple transport model. Relative to predictions, micropollutant sorption was decreased up to eightfold in acetogenic leachate, but increased up to sixfold in methanogenic leachate. This behaviour reflects a combination of interactions between the micropollutants, leachate DOM and aquifer mineral fraction. Sorption of DOM was not significant. Degradation of organic fractions occurred under Mn-reducing and SO4-reducing conditions. Degradation of some micropollutants occurred exclusively under Mn-reducing conditions. DOM and benzene were not significantly degraded under the conditions and time span (up to 280 days) of the experiments. Most micropollutants were degraded immediately or after a lag phase (32–115 days). Micropollutant degradation rates varied considerably (half-lives of 8 to >2000 days) for the same compounds (e.g., TeCE) in different experiments, and for compounds (e.g., naphthalene, DCB and TeCA) within the same experiment. Degradation of many micropollutants was both simultaneous and sequential, and inhibited by the utilisation of different substrates. This mechanism, in combination with lag phases, controls micropollutant degradation potential in these systems more than the degradation rate. These aquifer materials have a potentially large capacity for in situ bioremediation of organic pollutants in landfill leachate and significant degradation may occur in the Mn-reducing zones of leachate plumes. However, degradation of organic pollutants in acetogenic leachate may be limited in aquifers with low pH buffering capacity and reducible Mn oxides. Contaminants in this leachate present a greater risk to groundwater resources in these aquifers than methanogenic leachate.