Will our traditions for groundwater sampling in agricultural settings survive the 21st century?
Deltares, Department of Subsurface and Groundwater
For the evaluation of policy action programs to improve groundwater quality in agricultural catchments, research institutes and governments intensively monitor concentrations of agrochemicals in shallow or near surface groundwater. Conventionally, groundwater is sampled from irrigation or drinking water wells, specially designed groundwater sampling wells or temporary open boreholes. The samples are transported to laboratories and analysed. These conventional procedures are time consuming, expensive, and sometimes unsuitable for the assessment of groundwater quality status and trends. The spatial extent of point sampling from vertical wells and boreholes usually does not align with the scale of our research questions. In addition, the common yearly sampling intervals for upper groundwater do not capture short term weather induced variations in concentrations (Rozemeijer et al, 2009). This causes uncertainty in the groundwater quality status assessment and hampers the detection of trends. Innovative monitoring technologies, like chemical sensors and passive samplers may enhance groundwater quality research. High frequency groundwater quality monitoring using sensors reveals the temporal variability in upper groundwater quality. This reveals the groundwater response to groundwater recharge events as well as diurnal and seasonal cycles. Passive samplers produce time-integrated concentration values at low levels of detection.As long as a detailed assessment of field scale spatial heterogeneity of concentrations in upper groundwater is not the monitoring purpose, a conventional vertical groundwater well may not be to best option. Sampling from a horizontal well, or from a subsurface tile drain that taps the upper groundwater, has proven to yield a spatially field-scale integrated sample of upper groundwater (Rozemeijer et al., 2010). Modern horizontal drilling technology enables the installation of horizontal drains or groundwater monitoring wells at any depth or in any spatial configuration without disturbing the soil structure. Sensors, auto sampers, and passive samplers can be installed at the drain outlet for continuous monitoring and time or flow averaged concentration measurements. Distributed fibre optic sensing has been applied for temperature monitoring at high temporal and spatial resolutions. Current developments in chemical sensing using fibre optic technology may yield detailed distributed groundwater quality data. Innovations in groundwater quality monitoring are retarded due to costs, standardization, and the fear of artificial discontinuities in time series. Groundwater monitoring experimental ‘playgrounds’ and demonstration sites may accelerate the introduction of new technology. After field trials and comparison to conventional monitoring techniques, the application of appropriate innovations can be scaled up. Rozemeijer, J.C., Broers, H.P., Van Geer, F.C. & Bierkens, M.F.P., 2009: Weather-induced temporal variations in nitrateconcentrations in shallow groundwater. – J. Hydrol. 378: 119 –127.Rozemeijer J.C., Van der Velde, Y., Van Geer, F.C., Bierkens, M.F.P. & Broers H.P., 2010: Direct measurements of the tiledrain and groundwater flow route contributions to surface water contamination: From field-scale concentration patternsin groundwater to catchment-scale surface water quality. –Environ. Pollut. 158: 3571– 3579.