Transient simulations of flow and transport in the Chalk unsaturated zone

Mathias, Simon A.; Butler, Adrian P.; Jackson, Bethanna; Wheater, H. S.

doi:10.1016/j.jhydrol.2006.04.010

Cited by 61 publications

(48 citation statements)

References 48 publications

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“…The objectives of this study are to determine whether pesticides (and their degradation products) can form stable complexes with colloidal material present in the soil zone, and if these complexes also exist in groundwater beneath a relatively deep unsaturated zone. By considering our findings in relation to recently published field studies (Haria et al, 2003;Ireson et al, 2006) and modelling investigations (Mathias et al, 2005;Mathias et al, 2006), we present two conceptual models for pesticide movement from the point of agricultural application to the groundwater beneath.…”

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The significance of colloids in the transport of pesticides through Chalk

Gooddy

Mathias

Harrison

et al. 2007

Science of The Total Environment

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Agrochemical contamination in groundwater poses a significant long term threat to water quality and is of concern for legislators, water utilities and consumers alike. In the dual porosity, dual permeability aquifers such as the Chalk aquifer, movement of pesticides and their metabolites through the unsaturated zone to groundwater is generally considered to be through one of two pathways; a rapid by-pass flow and a slower 'piston-flow' route via the rock matrix. However, the dissolved form or 'colloidal species' in which pesticides move within the water body is poorly understood. Following heavy rainfall, very high peaks in pesticide concentration have been observed in shallow Chalk aquifers. These concentrations might be well explained by colloidal transport of pesticides. We have sampled a Chalk groundwater beneath a deep (30 m) unsaturated zone known to be contaminated with the pesticide diuron. Using a tangential flow filtration technique we have produced colloidal fractions from 0.45µm to 1kDa. In addition, we have applied agricultural grade diuron to a typical chalk soil and created a soil water suspension which was also subsequently fractionated using the same filtration system. The deep groundwater sample showed no evidence of association between colloidal material and pesticide concentration. In comparison, despite some evidence of particle trapping or sorption to the filters, the soil water clearly showed an association between the <0.45µm and <0.1µm colloidal fractions which displayed significantly higher pesticide concentrations than the unfiltered sample.Degradation products were also observed and found to behave in a similar manner to the parent compound. Although relatively large colloids can be generated in the Chalk soil zone, 2 it appears transport to depth in a colloidal-bound form does not occur. Comparison with other field and monitoring studies suggests that rapid by-pass flow is unlikely to occur beneath 4-5 m. Therefore, shallow groundwaters are most at risk from rapid transport of high concentrations of pesticide-colloidal complexes. The presence of a deep unsaturated zone will mean that most of the colloidal-complexes will be filtered by the narrow chalk pores and the majority of pesticide transport will occur in a 'dissolved' form through the more gradual 'piston-flow' route.

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“…However, through using a transient flow and transport, dualpermeability model and examining the effects of diffusion in the Chalk unsaturated zone, 11 Mathias et al (2005Mathias et al ( , 2006 showed that, at depth, almost all of the flow in the unsaturated zone was in the matrix.…”

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The significance of colloids in the transport of pesticides through Chalk

Gooddy

Mathias

Harrison

et al. 2007

Science of The Total Environment

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“…Notwithstanding the existence of a certain amount of 'bypass' flow in the Chalk unsaturated zone (e.g. Mathias et al, 2006), it is known from lysimeter studies that by 5 m below ground surface, waters are well-mixed chemically and isotopically (Darling and Bath, 1988;Van den Daele et al, 2007). This, combined with the Chalk's high porosity and a capillary fringe typically extending many tens of metres above the water table (Price et al, 1993), implies that water quality is very largely fixed before the water even reaches the water table, except for carbonate-system effects such as the sharp rise in alkalinity observed for some springs during the early part of the recovery, perhaps related to temporarily-elevated pCO 2 due to the onset of recharge.…”

Section: Water Qualitymentioning

confidence: 99%

The hydrochemistry of a Chalk aquifer during recovery from drought

Darling

Gooddy

Morris

et al. 2012

QJEGH

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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...

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“…Price et al, 2000;Haria et al, 2003) and physics-based (e.g. Mathias et al, 2006;Ireson et al, 2009) models have been proposed previously to describe water flow through the chalk unsaturated zone. The physics-based models mentioned above were developed based on a dualcontinua approach and required relatively large numbers of parameters (i.e.…”

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confidence: 99%

Towards a simple representation of chalk hydrology in land surface modelling

Rahman

Rosolem

2017

Hydrol. Earth Syst. Sci.

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Abstract. Modelling and monitoring of hydrological processes in the unsaturated zone of chalk, a porous medium with fractures, is important to optimize water resource assessment and management practices in the United Kingdom (UK). However, incorporating the processes governing water movement through a chalk unsaturated zone in a numerical model is complicated mainly due to the fractured nature of chalk that creates high-velocity preferential flow paths in the subsurface. In general, flow through a chalk unsaturated zone is simulated using the dual-porosity concept, which often involves calibration of a relatively large number of model parameters, potentially undermining applications to large regions. In this study, a simplified parameterization, namely the Bulk Conductivity (BC) model, is proposed for simulating hydrology in a chalk unsaturated zone. This new parameterization introduces only two additional parameters (namely the macroporosity factor and the soil wetness threshold parameter for fracture flow activation) and uses the saturated hydraulic conductivity from the chalk matrix. The BC model is implemented in the Joint UK Land Environment Simulator (JULES) and applied to a study area encompassing the Kennet catchment in the southern UK. This parameterization is further calibrated at the point scale using soil moisture profile observations. The performance of the calibrated BC model in JULES is assessed and compared against the performance of both the default JULES parameterization and the uncalibrated version of the BC model implemented in JULES. Finally, the model performance at the catchment scale is evaluated against independent data sets (e.g. runoff and latent heat flux). The results demonstrate that the inclusion of the BC model in JULES improves simulated land surface mass and energy fluxes over the chalk-dominated Kennet catchment. Therefore, the simple approach described in this study may be used to incorporate the flow processes through a chalk unsaturated zone in large-scale land surface modelling applications.

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Transient simulations of flow and transport in the Chalk unsaturated zone

Cited by 61 publications

References 48 publications

The significance of colloids in the transport of pesticides through Chalk

The significance of colloids in the transport of pesticides through Chalk

The hydrochemistry of a Chalk aquifer during recovery from drought

Towards a simple representation of chalk hydrology in land surface modelling

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