The Enfield-Haringey artificial recharge scheme comprises 23 abstraction/recharge boreholes located between Enfield and Haringey in north London. It is a strategic groundwater development aimed at increasing drought yields to meet the 'target level of service' defined by OFWAT for availability of water resources. The hydrogeology of the scheme, revealed by the drilling and testing of 15 new production boreholes is described and its strategic use summarized. The hydrochemistry of the groundwater in the Chalk and Basal Sands aquifer is briefly considered and the likely effects of re-saturation after recharge on the quality of the pumped groundwater is commented on.
This and previous studies have led to a better understanding of the aquifer system and the hydraulic relationship between the Basal Sands and the underlying Chalk. Intensive exploitation in the past led to some dewatering but reduction in abstraction since the 1950s has allowed water levels to recover, a process that is continuing. In most of the Enfield-Haringey area the aquifer is still only partially saturated, hence the potential for artificial recharge. The Chalk aquifer has been found to be typically anisotropic and heterogeneous with the upper 30–40 m being productive. Acidization was found to produce significant increases in well yields. Chalk groundwaters are mainly of a calcium bicarbonate type while those of the overlying Basal Sands are dominated by sulphate. Both groundwaters become progressively richer in sodium at the expense of calcium from north to south, in the direction of groundwater flow and towards more deeply confining conditions.
Sampling for organic micropollutants on a regional scale is from existing public supply or production boreholes. These give good samples for analysis but they will be of mixed waters and may not indicate the true occurrence of organic compounds in the groundwater. The most widely applicable groundwater sampling methods from open observation boreholes in the UK are based on submersible pumps. Several systems are now in use but all have drawbacks when used for organic micropollutant sampling. The present systems, particularly when used in conjunction with straddle-packers, are suitable for studies of inorganic groundwater quality or organic pollution at concentrations above about lmg1
-1
. There are increased difficulties, however, when sampling for trace organic contaminants below concentrations of 1μg1
-1
because of the ease of cross-contamination by the organic materials so prevalent in our environment. Great care has to be taken during sampling to minimize the cross-contamination.
Groundwater pollution by organic material derived from the recharge of raw, untreated sewage effluent has been studied at the sewage treatment works at Whitchurch in Hampshire. This organic investigation consisted of two parts; the validation of the sampling and analytical techniques and the field investigation.
Sampling points for the survey included open boreholes lined with uPVC or mild steel as well as specially designed buried in-situ groundwater samplers of uPVC or stainless steel construction. Laboratory leaching tests show significant pollution of sampled water by nylon and uPVC components and indicate that all plastics must be avoided in sample point construction. The in-situ samplers with stainless steel bodies and PTFE sampling tubes proved satisfactory.
The organic field survey showed that the organic pollution plume is coincident with the inorganic plume from the effluent recharge site. The total organic content of the effluent is reduced through infiltration; 85 per cent is removed in the unsaturated zone and a further 14 per cent during 300 m of lateral flow. The number of organic compounds at significant concentrations also is greatly reduced through infiltration. The most persistent micropollutant identified as coming solely from the sewage is dichlorobenzene which is present at about 9 µg/l below the recharge lagoons and 1 µg/l in the pollution plume 300 m from the site.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.