Presentations 2016

Groundwater nitrate concentrations in the Permo-Triassic aquifer of the Eden Valley, UK

British Geological Survey

https://www.youtube.com/watch?v=HVrKosUz2Sg&index=1&list=PLZC-5vj9_JAaQNkeKUtDhYOqJ3VZUX1BR

Groundwater nitrate concentrations in the Permo-Triassic aquifer of the Eden Valley, UK vary from less than 4 mg/l to in excess of 100 mg/l (as NO3). The variability is presumed to be due to land use and the main source of the nitrate is believed to be the nitrogen applied to grassland, both as slurry and as inorganic fertilizers. The main aim of this study was to estimate recharge rates and the timescale for water movement through the unsaturated zone and identify possible land use changes to reduce this influx of nitrate. Given the inherent uncertainties and limitations associated with the various methods for estimating recharge, it was proposed to use three different and independent methods and to compare the results obtained.The three methods proposed were:(i) to date the pore water profile within the unsaturated zone using the historical tracer tritium(ii) to date the pore water profile within the unsaturated zone using nitrate and chloride, released from the soil following the change in land-use from rough grazing to intensive pasture.(iii) To estimate recharge using a soil moisture water balance approach.The average recharge rate was found to be in the range 424-468 mm/y and the rate of water movement through the unsaturated zone is c. 3.5-3.85 m/y. Based on this estimate of water movement in the unsaturated zone, the travel time for recharge to migrate from the soil to the water table (or the delay imposed by the unsaturated zone) over the highest ground (where unsaturated zone thickness can be in excess of 175m) the travel times could exceed 50 years. However, over large areas of the Eden valley, the recharge currently arriving at the water table is of post 1990 origin. Thus an important conclusion is that, over most of the Eden valley, nitrate concentrations arriving at the water table are unlikely to substantially Increase but the effects of any BMPs are unlikely to be seen in the saturated zone for a significant number of years.