Assessing the effectiveness of a three-stage on-farm biobed in treating pesticide contaminated wastewater

Highlights

• Contaminated machinery washings were contained in an enclosed wash-down facility.

• A lined compost-straw-topsoil biobed removed 68–98% of individual pesticides.

• Drainage field irrigation removed a further 68–99% of residual pesticide.

• Mean total pesticide concentrations were 57 μg L⁻¹ after drainage field irrigation.

• Evidence of pesticide accumulation at depth within the drainage field.

Abstract

Agricultural point source pesticide pollution arising from contaminated machinery washings and accidental spillages pose a significant threat to river water and groundwater quality. In this study, we assess the effectiveness of a three-stage on-farm biobed for treating pesticide contaminated wastewater from a large (20 km²) commercial arable estate. The facility consisted of an enclosed machinery wash-down unit (stage 1), a 49 m² lined compost-straw-topsoil biobed (stage 2), and a 200 m² drainage field with a trickle irrigation system (stage 3). Pesticide concentrations were analysed in water samples collected fortnightly between November 2013 and November 2015 from the biobed input and output sumps and from 20 porous pots buried at 45 cm and 90 cm depth within the drainage field. The results revealed that the biobed removed 68–98% of individual pesticides within the contaminated washings, with mean total pesticide concentrations reducing by 91.6% between the biobed input and output sumps. Drainage field irrigation removed a further 68–99% of individual pesticides, with total mean pesticide concentrations reducing by 98.4% and 97.2% in the 45 cm and 90 cm depth porous pots, respectively. The average total pesticide concentration at 45 cm depth in the drainage field (57 μg L⁻¹) was 760 times lower than the mean concentration recorded in the input sump (43,334 μg L⁻¹). There was no evidence of seasonality in the efficiency of biobed pesticide removal, nor was there evidence of a decline in removal efficiency over the two-year monitoring period. However, higher mean total pesticide concentrations at 90 cm (102 μg L⁻¹) relative to 45 cm (57 μg L⁻¹) depth indicated an accumulation of pesticide residues deeper within the soil profile. Overall, the results presented here demonstrate that a three-stage biobed can successfully reduce pesticide pollution risk from contaminated machinery washings on a commercial farm.

Introduction

The widespread use of pesticides in agriculture to kill plant and insect pests which would otherwise reduce crop yields has been instrumental in enhancing global agricultural productivity since the mid-20th century (Oerke and Dehne, 2004, Oerke, 2005, Clarke et al., 2011; Popp et al., 2013). However, the harmful environmental impacts of applying toxic chemicals across large areas of the planet’s surface, particularly on the aquatic environment, are coming under increasing scrutiny (Skinner et al., 1997; DeLorenzo et al., 2001, Schwarzenbach et al., 2010). High profile cases, such as the effect of the insecticide DDT on the hatching success of raptors in the 1960s and 1970s, brought into focus the potential for pesticides to bio-accumulate through the food chain and negatively impact upon non-target species (Ames, 1966; Connell, 1988, Arnot and Gobas, 2006). Similarly, recent research has linked the use of neonicotinoid insecticides to the decline of bee populations in Europe and North America (Blacquiere et al., 2012, Whitehorn et al., 2012). Studies have also highlighted the significant economic costs associated with removing pesticides from drinking water. Between 1991 and 2000, water companies in the United Kingdom spent £2 billion treating pesticide contaminated water supplies (Jess et al., 2014), whilst in the United States the deleterious impacts of pesticide use were estimated to cost $9.6 billion in 2005 alone (Pimentel, 2005).

In order to tackle pesticide pollution, a range of national and international legislation is currently in force. Under the EU Water Framework Directive (2000/60/EC), specifically the Drinking Water (98/83/EC) and Groundwater (2006/118/EC) Directives, European Union member states must ensure that no individual pesticide concentration in drinking water at the tap exceeds 0.1 μg L⁻¹ and total pesticide concentrations should not exceed 0.5 μg L⁻¹. Additionally, the Pesticides Framework Directive (2009/128/EC) aims to reduce the damage caused by pesticides through the adoption of sustainable usage practices. In the United States, similar legislation exists under the Safe Drinking Water Act (1974) which places individual concentration limits on specific pesticides.

Pesticide pollution can either arise from diffuse sources, such as spray drift, leaching and overland flow, or from point sources, such as accidental spillages, leakages from equipment or from contaminated machinery washings (Carter, 2000, De Wilde et al., 2007). Whilst diffuse sources can in part be reduced by behavioural changes, such as timing of spraying to avoid periods of wet and windy weather to limit pesticide mobility, biobeds have emerged as a potentially important mitigation strategy for dealing with point source pollution (Fogg et al., 2003a, Reichenberger et al., 2007, Karanasios et al., 2010, Omirou et al., 2012).

The biobed concept originated in Sweden in the 1990s as a way of using microbial activity to degrade waste pesticide residues (Torstensson, 2000). A biobed is essentially a moderately sized pit (typically tens of cubic metres in volume) which can be lined or unlined and is filled with a 1:2:1 matrix of compost, straw and topsoil. The surface is covered with grass and onto this the waste pesticide residues are deposited. In principle, microorganisms (e.g. bacteria and fungi) within the biobed matrix chemically and physically interact with the pesticides leading to structural changes and/or complete degradation (Pinto et al., 2016). To work effectively, the biobed mixture needs to have high pesticide absorption capacity and be able to facilitate high rates of microbial activity (Castillo et al., 2008). For this reason, straw is included to enhance microbial activity, particularly that of lignin-degrading fungi (e.g. white rot fungi) which produce phenoloxidase enzymes that have a broad specificity and are thereby able to degrade a wide range of pesticide residues (Bending et al., 2002). Soil is included to increase the sorption capacity of the matrix material so that it holds onto the pesticides and also provides a source of microorganisms for biodegradation. Lastly, compost is added to increase sorption capacity, improve moisture content and decrease the pH to make conditions favourable for fungi growth. The surface grass layer aids water regulation and prevents surface crusting, thus limiting the formation of cracks that would open up preferential pathways for pesticides to escape the biobed prior to degradation (Fogg et al., 2004, Castillo and Torstensson, 2007, Castillo et al., 2008). In lined biobed systems, common in the United Kingdom (UK), the leachate is typically collected from the bottom of the biobed and re-used for either irrigation, sprayer washing or as a carrier for further herbicide applications. Irrigation can be on infield crops or a designated drainage area. In order to minimise pollution risk and comply with UK environmental protection legislation, the drainage area must be vegetated, be neither frozen or water logged, be > 10 m away from any surface waterbody, be > 50 m from any spring, well or borehole not used for domestic supply or food production, and be > 250 m away from any borehole that is used for domestic supply or food production (Environment Agency, 2007).

Established in 2010, the River Wensum Demonstration Test Catchment (DTC) project is a part of a UK government funded initiative to evaluate the extent to which on-farm mitigation measures can be employed to cost effectively reduce the impacts of agricultural pollution on river ecology whilst maintaining food production capacity (Outram et al., 2014). Draining a catchment area of 660 km² in Norfolk, UK, of which ∼63% is arable land, the River Wensum supplies drinking water for the city of Norwich and is affected by agricultural pesticide pollution. A small unpublished water quality monitoring study carried out at 20 locations on the River Wensum over a 16-week period in autumn 2012, revealed that 23% of samples contained individual pesticide concentrations greater than the 0.1 μg L⁻¹ drinking water limit. Five key pesticides (metaldehyde, metazachlor, dimethenamid, flufenacet and propyzamide) accounted for 90% of all detected compounds, with 21% of samples containing metaldehyde concentrations >1 μg L⁻¹ (further details of this study can be found in the electronic supplementary material). Partly in response to this pesticide pollution pressure, an on-farm biobed unit capable of treating contaminated machinery washings was installed at Manor Farm, Salle, in the Blackwater sub-catchment of the River Wensum. This was part of a trial package of on-farm mitigation measures, co-funded under the Catchment Sensitive Farming (CSF) initiative (Natural England, 2014), aimed at reducing agricultural pollution.

The primary objectives of this paper are as follows:

• To assess the efficiency of the Manor Farm biobed at reducing pesticide concentrations in agricultural machinery washings;

• To assess the effectiveness of drainage field irrigation at further reducing pesticide concentrations in biobed leachate;

• To determine if biobed pesticide removal is more efficient for certain types of pesticide;

• To assess temporal variability in the effectiveness of the biobed.