Ploughing up permanent pasture may cause elevated nitrate concentrations in public groundwater supply abstractions

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As part of a study into nitrate transport within groundwater catchments, for United Utilities, we* noted some interesting features in nitrate concentration trends of groundwater abstraction boreholes around Delamere, Cheshire. Most of the material in this post was delivered, on behalf of ESI Ltd, by Dr Steve Buss at the Meres and Mosses Forum conference, at Cholmondeley Castle, on 20 September 2013.

We identified catchments for, and reviewed nitrate concentration trends in, seven of United Utilities’ abstraction boreholes in the area. The aquifer is unconfined Sherwood Sandstone and the land is mostly used for dairy farming. (A full explanation of how this was achieved is written up here, in a paper that Dr Steve Buss gave to the Irish Branch of the International Association of Hydrogeologists in April 2013.) Modelled trends for the three boreholes with the largest catchments fitted the data very well (e.g. Figure 3 in the paper in the link above). But in the four with the smallest catchments, we saw that the models considerably underestimated recent concentration data (below). We hadn’t observed anything like this in any of the other 37 catchments that we’ve looked at; and the close proximity of the catchments suggested a common origin for these peaks.

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So what did this excess nitrate look like as it came out of the soil zone? As part of the trending process we estimated a travel time for each catchment. So we can calculated the mass of excess nitrate (i.e. concentration above trend line times the abstraction rate) and shift that back to the time around when it would have come out of the soil zone (below – point colours correspond to those above). This clearly suggests a common start date for these peaks – apparently around the mid to late 1970s.

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This was a period of intensification of the dairy industry in Cheshire. Animals were being housed more, and pasture was converted to maize growing for fodder. Under maize, about twice as much nitrate gets through the soil zone as under grass (c. 180 mg/l vs. 70 mg/l; ADAS 2011). This was one important change that would have let to an increase in groundwater nitrate, but not the only one.

The major cause of these peaks seems to be ploughing up permanent pasture, which releases a lot of nitrate from the soil. Under non-ploughed pasture, organically-bound nitrogen builds up in the soil over time. Ploughing exposes that organic matter to the atmosphere and the insoluble organic-bound nitrogen is mineralised to soluble nitrate. It then gets washed through the soil to groundwater over the next few infiltration seasons. Whitmore et al., (1992) found that in the first season after ploughing up permanent pasture, water leaving the soil zone may have a nitrate concentration of around 450 mg/l. This may drop to around 225 mg/l after five years, and then to about 100 mg/l (less than the contribution from arable farming) after about another five years.

So how might this affect the Meres and Mosses Nature Improvement Area? The analysis above shows that, in a small groundwater catchment, ploughing up permanent pasture can be a significant cause of nutrient release. This is just another reason that ploughing permanent pasture should be discouraged in the vicinity of sensitive environmental receptors.

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(As an aside, travel time correlated very well with catchment size – right. This simple analysis shows that the velocity of nitrate through the 50 m thick unsaturated zone is about 2 m/year here, and groundwater velocity is about 0.35 km/year.)

* These findings result from work by Dr Steve Buss, Lawrence Brown (Hafren Water), David Johnson (ADAS), Chrystina Bemment and Dr Simon Arthur (ESI).

References

ADAS, 2011. Nitrates Directive Consultation Document The evidence base for assessing the impacts of the NVZ Action Programme on water quality across England and Wales.

Whitmore, A.P., Bradbury, N.J. and Johnson, P.A., 1992. Potential contribution of ploughed grassland to nitrate leaching. Agriculture, Systems and Environment 39, 221-233.