J.W. Lloyd

Review of natural and artificial denitrification of groundwater

Abstract An approach to securing low nitrate supplies is to take advantage of the bacterially mediated process of denitrification. This paper reviews the factors controlling denitrification and describes the evidence for denitrification in the groundwater environment. It is found that natural denitrification, although not extensive, does occur in a variety of aquifers where there is a sufficient source of oxidizable organic carbon. However, natural processes cannot be expected to remove the large quantity of nitrate now present in many aquifers. Water treatment processes that stimulate artificial denitrification by the injection of the required nutrients are under development, and may offer a simple and inexpensive method of nitrate removal. This paper reviews the various above-ground and underground techniques to determine the advantages and disadvantages of this approach compared with other nitrate removal technology. It is concluded that artificial denitrification as an above-ground process affords the best nitrate removal rates and process control. Compared with alternative technology, namely ion exchange, above-ground biological treatment does not generate large quantities of waste product, but is more difficult to automate. Underground processes have been developed, but their long-term success will depend on restricting blocking of the aquifer matrix with accumulated biological matter.

An example of the use of factor analysis and cluster analysis in groundwater chemistry interpretation

Abstract Factor analysis and cluster analysis, as applied to two widely differing sets of groundwater hydrochemical data, appear to be moderately successful as statistical tools for revealing hydrochemical and hydrogeological features, including patterns of groundwater flow. They possess advantages over the traditional graphical methods of solving similar problems, principally in their systematic nature, and they can generate inter-parameter relationships that may be overlooked in the less sophisticated traditional methods. Their disadvantages are that they are easily prone to misuse and misinterpretation due to their complexity. Further, there is a need for the user to have adequate statistical knowledge.

The sensitivity of parameters in the Penman evaporation equations and direct recharge balance

Abstract The various parameters in the Penman equations and direct recharge balance are varied to determine their influence on the calculated evaporation and recharge amounts. The sensitivity analysis is applied to data from an area of the north Lincolnshire Chalk. Evaporation estimates are found to be particularly sensitive to albedo, radiation constants and wet-bulb temperature. The resultant differences in turn significantly affect recharge estimates. The study also indicates the need to use daily computations of recharge in place of the normally used ten-daily or monthly computations.

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.

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.

Modelling non-point sources of nitrate pollution of groundwater in the Great Ouse Chalk, U.K.

Abstract A numerical distributed transport model, is described which was used to simulate nitrate concentrations in groundwater. The model takes into account the nitrate input from the soil zone and the movement of nitrate in the unsaturated and saturated zones. The model was developed for an area of 600 km 2 of the Cretaceous Chalk, near Cambridge, England. The model area is described in terms of geology, hydrogeology, land use and agricultural practice. The Chalk aquifer was represented by rectangular cells which are divided into an upper and lower layer to represent the unsaturated and saturated zone. The procedure adopted to calculate the nitrate input from the soil zone is outlined. A simplified equation was used to account for the delay for nitrate to pass through the unsaturated zone. Nitrate concentrations in groundwater were determined using a mass-balance approach. The model results in the simulation mode and in the forecast mode are presented and discussed. The sensitivity of the model is examined.

Controls on silica in groundwater environments in the United Kingdom

Abstract The nature and behaviour of silica has been investigated in the shallow groundwaters of the United Kingdom. Silicon exists in solution as the monomer Si(OH)4. It is a stable species and chemically unreactive showing no affinity for other major dissolved constituents under normal groundwater conditions. Silica concentrations in British groundwaters range up to 30 mg l−1 SiO2 with an input level in recharge waters of 5 mg l−1 SiO2. The input concentration is derived from hydrolysis of silicates and the solution of siliceous minerals within the soil zone. Detailed investigations of silica in British aquifers has shown that the development of observed silica levels reflects the degree to which reactions between groundwater and silicate minerals in the aquifer matrix are allowed to take place. The final concentration is attained when the groundwater reaches equilibrium with the least stable silicate. This is illustrated with details from the Chalk aquifer of Lincolnshire. Silica concentrations are conservative and may be diagnostic of groundwater bodies, and variations of a few mg l−1 may be regarded as significant. Interpretation of silica levels requires the identification of hydrochemical processes that may influence the water-rock reactions and the final equilibrium between the groundwater and silicate minerals.

The hydrogeology and groundwater resources development of the Cambro-Ordovician sandstone aquifer in Saudi Arabia and Jordan

Abstract The Saq Cambro-Ordovician sandstones form the major national aquifer in Saudi Arabia and Jordan. Significant abstraction of groundwater has taken place since the early 1950s and monitoring of the aquifer responses indicates a moderate transmissivity but high specific yield. The groundwater quality is excellent for irrigation which is the present major use, and is also good for potable purposes. Although large drawdowns are being experienced locally in confined areas of the aquifer the unconfined area responses show the aquifer to have an excellent potential for development under groundwater mining principles that may be considered for use outside the areas of occurrence.

A study of uranium and thorium series isotopes in Chalk groundwaters of Lincolnshire, U.K.

A groundwater investigation is described in which uranium and thorium series radionuclides are applied to the study of the hydrogeologically well-documented Lincolnshire Chalk aquifer system of North Lincolnshire, England. The dominance of Eh in controlling the uranium content of groundwaters is discussed, and the 230Th234U activity ratio used as an indicator of the speciation of uranium. In highly reduced waters the 230Th234U activity ratio approaches unity, suggesting that the uranium in solution is behaving similarly to thorium in the +4 valence state. In the oxidised environment the higher uranium content groundwaters exhibit lower 234U238U activity ratios. A model of mixing young, low 234U238U activity ratio fissure waters with old, high 234U238U activity ratio pore waters is discussed, but rejected. It is considered that preferential dissolution of 234U in the lower rate weathering regimes produces the observed relationship between uranium content and activity ratio. A method of in-situ ^Rn (random release factor) determination is suggested; however, the calculated values for ^Rn are disproportionately high for a fissure flow system. This, along with evidence of a radium depleted aquifer matrix indicates that radium is being mobilised and transported from the pore water system to the fissure system, and subsequently adsorbed by clay minerals lining the Chalk fissures.