Atul H. Haria

Penetration of herbicides to groundwater in an unconfined chalk aquifer following normal soil applications

The persistence and penetration of the herbicides isoproturon and chlorotoluron in an unconfined chalk aquifer has been monitored over a 4-year period through soil sampling, shallow coring and groundwater monitoring. Chlorotoluron was applied on plots as a marker compound, having never been used previously on that, or surrounding fields. The fieldsite had a 5 degree slope with soil depths of 0.5 to 1.5 m and a water table between 20 and 5 m from the soil surface. Where the water table was deepest (9-20 m below surface (mbs)) little or no positive herbicide detections were made. However, where the water table was at only 4-5 mbs, a regular pesticide signal of around 0.1 microg/l for isoproturon and chlorotoluron could be distinguished. Over the winter recharge period automatic borehole samplers revealed a series of short-lived peaks of isoproturon and chlorotoluron reaching up to 0.8 microg/l. This is consistent with a preferential flow mechanism operating at this particular part of the field. Such peaks were occurring over 2 years after the last application of these compounds. Shallow coring failed to uncover any significant pesticide pulse moving through the deep unsaturated zone matrix at the fieldsite.

Preferential Flow Pathways and Their Capacity to Transport Isoproturon in a Structured Clay Soil

A field experiment was established to monitor preferential flow pathways and their capacity to transport isoproturon in a heavy clay soil. A hydrologically defined plot of 600 m 2 at a field site on the Oxford University Farm at Wytham was created with integral flow monitoring and sampling devices. Data are presented from two flow events which occurred in April and May 1994. The highest concentrations of isoproturon (130 μg litre -1 ) were observed in the drainage system. The vast majority of the 0.7% of applied pesticide that left the plot was via the drainage system (75-90%) with lateral subsurface flow accounting for a smaller proportion (max 23%). Whilst high pesticide concentrations could be found in overland-flow water, the volumes of water moved by this route were small (max 3%). Less water was estimated to have left the field in response to rainfall than in the previous year. This was attributed to decay of the mole drain system. Consequently the amount of applied pesticide lost in runoff (0.7%) was less than that estimated for the first year (1.5%). The work has shown that, even when a farmer follows best practice in the application of a herbicide to a winter cereal in a drained clay field, high concentrations of the herbicide (relative to the EC drinking water limit) will contaminate surrounding watercourses.

Water movement and isoproturon behaviour in a drained heavy clay soil: 1. Preferential flow processes

Abstract The processes and mechanisms that control pesticide transport from drained heavy clay catchments are being studied at Wytham Farm (Oxford University) in southern England. In the first field season field-drain water contained high concentrations of pesticide. Soil studies demonstrated that the main mechanism for pesticide translocation was by preferential flow processes, both over the soil surface and through the soil profile via a macropore system that effectively by-passed the soil matrix. This macropore system included worm holes, shrinkage cracks and cracks resulting from ploughing. Rainfall events in early winter rapidly created a layer of saturation in the A horizon perched above a B horizon of very low hydraulic conductivity. Drain flow was initiated when the saturated layer in the A horizon extended into the upper 0.06m of the soil profile; thereafter water moved down slope via horizontal macropores possibly through a band of incorporated straw residues. These horizontal pathways for water movement connected with the fracture system of the mole drains, thus feeding the drains. Overland flow occurred infrequently during the season.

Water movement and isoproturon behaviour in a drained heavy clay soil: 2. Persistence and transport

In a study of isoproturon applied to winter wheat in a heavy clay soil, high concentrations of herbicide were detected in overland flow (surface runoff) water, mole drain water and field drain water. The amount of isoproturon detected in the field drain, over two major rainfall events in 1993, was estimated to be 2.7% of that potentially available at the soil surface. The peak drain water concentration of isoproturon in the first significant drain flow event was in excess of 500 ppb. For these two rainfall events, 1% of the compound originally applied was lost to the drainage system. Comparison of isoproturon with chloride and sulphate concentrations suggests different origins for the drain water, with the majority of isoproturon being carried down to the drainage system by preferential flow from the soil surface. A residue of 5–10% of the herbicide persisted in the top soil and did not appear to be degraded in the period of May to June.

Flow regime effects on reactive and non-reactive solute transport

The lower boundary maintained throughout leaching experiments in column and lysimeter studies influences the transport behavior of solutes. In this study we wanted to determine the effects of different flux regimes and soil organic matter treatments on reactive (pesticide) and non-reactive (bromide) solute movement in lysimeters. Four large soil monoliths, collected from the same experimental site but differing in the organic carbon content by an added manure layer, were subjected to two consecutive leaching tests. Unsaturated steady-state flow with a unit hydraulic gradient was established in Test 1 by applying a constant negative pressure at the base of the lysimeter while during Test 2 atmospheric pressure was maintained (zero-tension). Although less herbicide (isoproturon) was leached from the soil with added manure, the flux regime, as controlled by the lower boundary, dominated the adsorption and the degradation coefficients. The lower moisture content of Test 1 enhanced isoproturon binding, whilst chemical degradation was more effective under the increased but non-uniform soil water content of Test 2. This study demonstrates how solute leaching studies using zero-tension lysimeters do not represent unsaturated in-field processes; the physicochemical processes occurring in unsaturated field soils are only adequately reflected in leaching tests when a constant negative soil water pressure head is maintained throughout the soil sample.

Bedrock Groundwater as a Major Control on Streamflow Generation in Upland Wales, UK

Hard rock upland regions source some of the largest rivers in the UK and therefore constitute an important water resource. The water quality of these headwater streams has been the subject of concern for a number of decades; the predominant focus in NW Europe has been in respect to catchment acidification and the resulting impact on stream ecology (e.g. Harriman and Morrison, 1982). The stream water quality response of these catchments to anthropogenic acid deposition is a function of the range of travel time distributions, resulting from differences in flow pathways, from when rainfall enters the catchment to its appearance in the stream channel (Kirchner et al., 2000, 2001). Understanding these flow pathways and the mechanisms of streamflow generation is fundamental for understanding the catchment response to pollution.