Roberts, Shawn Conrad; (1999) Surface/groundwater interactions affecting the Lincolnshire Limestone aquifer. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Lincolnshire Limestone, a regionally important aquifer in the UK, is in hydraulic continuity with rivers draining intensely-farmed arable land. To assess the impact of riverine chemistry on groundwater quality, nutrient concentrations were monitored in surface and ground waters over a two-year period. Fertiliser applications and heavy rain resulted in winter riverine nitrate concentrations up to 145 mg/l; sewage treatment works (STW) effluent lead to localised summer nitrate concentrations up to 186 mg/l. Rapid recharge to the aquifer, through swallow holes and river beds, occur year round. Plumes of nitrate-rich water are drawn through the aquifer by public supply abstraction boreholes, modifying the regional chemistry. Riverine concentrations of dissolved organic carbon (DOC) peaked in low-flow summer conditions (up to 14.6 mg/l), due to algal contributions and STW effluents. Concentrations were lower in winter (1.9-8.5 mg/l) due to dilution. Groundwater concentrations of DOC were 0.6-1.0 mg/l, with little regional variability, except for an increase to 2.0 mg/l in the deep confined zone. Recharging DOC was rapidly consumed by microbial processes, such as denitrification, but was insufficient in amount to account for much nitrate reduction in the aquifer. Other nitrate reduction mechanisms appeared to be autotrophic denitrification via pyrite oxidation, shown by increased iron and sulphate concentrations, and inorganic reduction of iron (II). The Lincolnshire Limestone has been naturally resisting fertiliser contamination, but rising concentrations of nitrate and sulphate indicate that the aquifer is becoming unable to cope with rising loads. It is, therefore, concluded that the confined aquifer is under threat from agricultural contamination.

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