A.T. Williams

A review of the impact of climate change on future nitrate concentrations in groundwater of the UK

This paper reviews the potential impacts of climate change on nitrate concentrations in groundwater of the UK using a Source-Pathway-Receptor framework. Changes in temperature, precipitation quantity and distribution, and atmospheric carbon dioxide concentrations will affect the agricultural nitrate source term through changes in both soil processes and agricultural productivity. Non-agricultural source terms, such as urban areas and atmospheric deposition, are also expected to be affected. The implications for the rate of nitrate leaching to groundwater as a result of these changes are not yet fully understood but predictions suggest that leaching rate may increase under future climate scenarios. Climate change will affect the hydrological cycle with changes to recharge, groundwater levels and resources and flow processes. These changes will impact on concentrations of nitrate in abstracted water and other receptors, such as surface water and groundwater-fed wetlands. The implications for nitrate leaching to groundwater as a result of climate changes are not yet well enough understood to be able to make useful predictions without more site-specific data. The few studies which address the whole cycle show likely changes in nitrate leaching ranging from limited increases to a possible doubling of aquifer concentrations by 2100. These changes may be masked by nitrate reductions from improved agricultural practices, but a range of adaption measures need to be identified. Future impact may also be driven by economic responses to climate change.

An empirical liquid permeability—gas permeability correlation for use in aquifer properties studies

Abstract Laboratory permeability studies can contribute significantly to the quantification of aquifer heterogeneity. However, intrinsic permeabilities obtained by standard core analysis techniques using gas are different from those obtained using water. This is because gas measurements may be affected by a molecular phenomenon known as gas slippage. An empirical correlation is presented for liquid and gas permeability measurements obtained for a suite of Permo-Triassic sandstones and shales from the Sherwood Sandstone Group of northern England. Liquid permeability tests were performed using synthetic formation brines and deionized water. Gas permeability tests used nitrogen as the permeant. Liquid permeabilites, kl, ranged from 9.0 x 10-19 m2to 2.4 x 10-12m2 and gas permeabilites, kg, ranged from 1.7 x 10-17m2 to 2.6 x 10-12m2.The liquid and gas permeability data exhibit log-normal frequency distributions; the log transformed liquid and gas permeability data have means of 5.1 x 10-16m2 and 4.3 x 10-15m2respectively. A linear least-squares fit to the data has the form log10kl = 1.17log10kg + 1.51. kl/kg ratios, in the range 0.03 to 0.9, indicate that Hagen-Poiseuille type models may not provide appropriate descriptions of gas flow in the Sherwood Sandstone.

Effectiveness of the Nitrate Sensitive Areas Scheme in reducing groundwater concentrations in England

The Nitrates Sensitive Areas (NSA) Scheme in England was a voluntary, compensated measure from 1990 to 2003 which aimed to reduce nitrate leaching from agricultural land to vulnerable groundwaters by modifying land use management. Measurements from 22 NSAs introduced in 1994/5 show an overall 34% decrease in the nitrate concentration of water leaching from the soils from 115 mg/l (1994/5-1995/6) to 76 mg/l (1998/9-1999/2000). This study looks at two NSAs in depth. The Old Chalford NSA consists of a small (81 km2) catchment with a series of spring sources in the Oolitic Limestone in Oxfordshire, whilst the Pollington NSA is the much larger (358 km2) catchment of three large public water supply sources (PWS) in the Sherwood Sandstone of North Yorkshire. Soil leaching model results suggest that the Scheme reduced root zone nitrate concentrations from 98 mg/l in 1994 to 69 mg/l in 1998 at Pollington NSA, and from 43 mg/l in 1990 to 37 mg/l in 1998 at Old Chalford NSA. These data served as inputs into flow modelling to quantify the effect of changes in the soil zone on groundwater concentration. At Old Chalford changes in the soil zone had a measurable effect at abstraction points after only two years, whereas Pollington NSA has shown little effect of the Scheme on abstracted groundwater concentration to date as the geology and geometry of the source catchment zones are expected to lead to a noticeable impact only after 30 years. Although results demonstrate the effectiveness of the Scheme in reducing root zone nitrate leaching, the timescales involved in groundwater responses mean that, in many areas, the impact of such pollution control measures will not be realized for several decades.

Recovering tracer test input functions from fluid electrical conductivity logging in fractured porous rocks

A radially convergent tracer test was carried out in an unconfined Chalk aquifer of Berkshire, United Kingdom. Fluorescent tracers were injected into two boreholes lying 32 m (PL10A) and 54 m (PL10B) from the abstraction hole. The tracers were also mixed with an NaCl solution so that vertical distributions of tracer within the injection wells could be monitored using fluid electrical conductivity (FEC) logging. The breakthrough curve (BTC) from PL10A was unimodal and had a first arrival time of 14 min. The BTC from PL10B exhibited two distinct peaks and a first arrival time of just 4 min. The tracer test input functions were derived by numerically modeling the observed FEC logs of the injection wells. These were then convoluted with a conventional, Fickian matrix diffusion dual-porosity model. The results suggested that the multiple peaks were due to the way in which the tracers left the injection wells and migrated into the aquifer. FEC log inversion proved to be an effective method for predicting borehole flow data obtained by flowmeters and recovering tracer test input functions for radially convergent tracer tests.

Geochemical modelling of fluoride concentration changes during Aquifer Storage and Recovery (ASR) in the Chalk aquifer in Wessex, England

During ASR-cycle testing at a site in the confined Chalk near Lytchett Minster in Dorset, the high concentration of fluoride in the recovered water posed severe limitations on the success of the scheme. Based on physical modelling, the dual porosity character of the Chalk combined with high fluoride concentrations in the native water were identified as the key factors controlling the measured concentrations. However, mixing of water between the matrix pores and fractures was not sufficient to explain the fluoride concentration and it was concluded that there was an additional release of fluoride from aquifer interaction. This led to an additional increase in the fluoride concentration in the recovered water. In order to investigate this hypothesis, a geochemical model incorporating reactions between the injected water, the native groundwater and the aquifer minerals was developed. The geochemical model PHREEQC-2 was set up so that it was capable of modelling ASR-cycles (including radial flow and diffusive mixing as a consequence of dual porosity). The physical aspects of the model were calibrated using a 3-D dual porosity transport model (SWIFT). Different geochemical processes (e.g. limited mineral availability, reaction kinetics) causing fluoride concentrations above those expected from dual porosity mixing were investigated. Comparing the modelled results with the observations from the test site suggested that slow dissolution of fluoride minerals (fluorite) was likely to be responsible for the additional increase in fluoride concentration in the recovered water.

A tracer methodology for identifying ambient flows in boreholes

Identifying flows into, out of, and across boreholes is important for characterizing aquifers, determining the depth at which water enters boreholes, and determining the locations and rates of outflow. This study demonstrates how Single Borehole Dilution Tests (SBDTs) carried out under natural head conditions provide a simple and cheap method of identifying vertical flow within boreholes and determining the location of in-flowing, out-flowing, and cross-flowing fractures. Computer simulations were used to investigate the patterns in tracer profiles that arise from different combinations of flows. Field tracer tests were carried out using emplacements of a saline tracer throughout the saturated length of boreholes and also point emplacements at specific horizons. Results demonstrated that SBDTs can be used to identify flowing fractures at the top and bottom of sections of vertical flow, where there is a change in vertical flow rate within a borehole, and also where there are consistent decreases in tracer concentration at a particular depth. The technique enables identification of fractures that might be undetected by temperature and electrical conductance logging, and is a simple field test that can be carried out without pumping the borehole.

An early warning system for groundwater flooding in the Chalk

Abstract An early warning system has been developed for groundwater flooding and trialled in the Patcham area of Brighton. It provides a fit-for-purpose approach for forecasting groundwater flood events in the Chalk and is capable of operating across longer time scales than had previously been possible. The method involves a set of nested steps or tasks. Initially, the catchments response to recharge is determined and, using a representative hydrograph, a simple regression model that relates annual groundwater level minima and autumn and winter rainfall to subsequent annual maxima is developed. The regression model is then applied at the end of each summer recession using the observed annual minimum and estimates of winter rainfall to predict the following groundwater level maximum. Based on the results of this prediction a variety of steps may then be appropriate. Where the model predicts potentially high groundwater levels the frequency of groundwater level monitoring observations can be increased. A novel element of the method developed is the monitoring of changes in the matric potential of the unsaturated zone. Specific trigger levels to initiate either the next step of the method or promulgation of warnings of varying severity will be developed through experience of use of the system.

Tracer Investigations of the Nature and Structure of Subsurface Voids in Mildly Karstic Aquifers: an Example from the English Chalk

The Chalk in England is a mildly karstified fractured limestone with high matrix porosity. Chalk landscapes are predominantly a fluviokarst with an extensive dry valley network and with stream sinks and dolines developed in zones close to overlying strata. The nature and extent of subsurface dissolutional voids is poorly understood and field tests using artificial tracers were carried out to investigate their structure. Tracer testing from stream sinks demonstrated rapid groundwater flow (up to 5 km/day) confirming previous Chalk tracer studies. However tracer attenuation was variable with extremely low tracer recoveries at some sites indicating that not all flowpaths fed by stream sinks comprise fully connected conduit systems. A conceptual model of flow along multiple flowpaths comprising a distributary network around stream sinks and a tributary network around springs with variable connectivity between the two is presented. The proposed conceptual model of flowpath structure in the Chalk resembles previous models of the early stages of speleogenesis. It is suggested that groundwater flow in the Chalk and other mildly karstic aquifers occurs within complex small scale dissolutional networks, in which there is a high degree of connectivity between larger conduits, fissures, and a primary fracture network. Advection from karstic channels into smaller voids results in high attenuation of solutes and particulates providing a degree of protection to groundwater outlets that is not seen in more highly karstic aquifers.