The hydrochemistry of a Chalk aquifer during recovery from drought

Abstract

Chalk groundwater levels typically decline markedly in response to drought, and rebound strongly when the drought breaks. Chalk streams, largely groundwater-fed, are of ecological importance but little research has been conducted on possible water-quality effects accompanying fluctuations in groundwater level. This study monitored springs, boreholes and surface water in the Pang and Lambourn catchments in southern England during a major recovery in 2006-08. Hydrochemistry, stable isotopes and age indicators were used to characterise the waters. Perennial springs showed little change in water quality over the monitoring period, and even seasonal springs soon became consistent in their hydrochemistry. A similar lack of change was observed in borehole waters and in the River Lambourn. Stable isotopes demonstrated the high degree of damping relative to rainfall inputs, while residence time indicators showed that Chalk groundwater is basically a mixture with an 'old' (pre-1950s) component of 50%. This being the case, any water quality changes due to water level fluctuations would inevitably become diluted. Therefore, although future climate predictions for southern Britain include greater extremes in rainfall and temperature, and consequently water level changes of greater amplitude, the buffering effect of the Chalk aquifer should protect the quality of Chalk springs and streams.The quality of Chalk groundwater, especially where it issues into surface watercourses from springs, is of some importance both from amenity and regulatory perspectives. With regard to the latter, the EU's Water Framework Directive requires that "good ecological status" and "good chemical status" be maintained or restored (Kallis and Butler, 2001). While the effects of future climate change on the water balance of southern England remain to be established in any detail, it seems likely that there will be more extremes in rainfall and temperature (Hulme et al., 2002). Groundwater flow in the Chalk aquifer occurs very largely through the fracture porosity. The development of this is related to base level changes in response to fluctuations in sea level, resulting in discrete flow horizons separated by much less permeable layers (Butler et al., 2009). This heterogeneous distribution of fracture porosity and permeability means that the aquifer can exhibit major changes in water level in a short period, leading to phenomena such as groundwater flooding (Macdonald et al., 2008) and the possibility of changes in water quality. At the same time, the Chalk's high primary porosity has been viewed as having a buffering effect on hydrochemistry (Barker and Foster, 1981).Although groundwater drought and recovery in the Chalk have received attention in several studies, the present paper is the first to go into significant detail on the water-quality aspects of the cycle. This study addresses the effect of a major rise in water level following an anomalously dry period, primarily by detailed monitoring of a range of spring outlets during the r...

Drought termination

“…Cornwall) continued to be below average. Marsh et al (2007) Darling et al (2012 2003 (2003) UK A sequence of Atlantic frontal systems brought substantial rain in the early winter. Soil moisture deficits were eradicated rapidly and infiltration returned river flows to the normal range by winter (although not rapidly).…”

A long‐term study of stable isotopes as tracers of processes governing water flow and quality in a lowland river basin: the upper Thames, UK

“…This raises the question of the extent to which groundwater isotopic signals are preserved in the various rivers of the catchment. The following groundwater quality studies carried out in the upper Thames region allow a comparison: Alexander and Andrews (), Bearcock and Smedley (), Cobbing et al (), Darling et al (), Edmunds et al (), Jackson et al () and Neumann et al ().…”

Groundwater conceptual models: implications for evaluating diffuse pollution mitigation measures

“…Typically waters issuing from the unconfined Chalk aquifer have δ 13 C-DIC values in the range -13 to -16‰ (e.g. Darling et al, 2012b), with more negative values suggesting the addition of CO 2 from the oxidation of organic material.…”

“…The use of CFCs and SF 6 as sensitive environmental tracers in groundwater investigations is outlined in Darling et al (2012a). All CFC-11 concentrations are abovemodern, indicating a pervasive small pollution-related enhancement typical of many groundwaters in southern England (Darling et al, 2012b). While a small amount of CFC-12…”