|Main authors:||Birgitte Hansen, Hyojin Kim, Ingelise Møller, Abel Henriot, Marc Laurencelle, Tommy Dalgaard, Morten Graversgaard, Susanne Klages, Claudia Heidecke and Nicolas Surdyk|
|FAIRWAYiS Editor:||Jane Brandt|
|Source document:||»Birgitte Hansen, Hyojin Kim, Ingelise Møller, Abel Henriot, Marc Laurencelle, Tommy Dalgaard, Morten Graversgaard, Susanne Klages, Claudia Heidecke and Nicolas Surdyk 2021. Evaluation of ADWIs: agri-drinking water quality indicators in three case studies (FAIRWAY Project Deliverable 3.2)|
|1. Nitrogen indicators|
|2. Pesticide indicators|
|3. Link indicators|
1. Nitrogen indicators
Based on the analyses from the Island Tunø, Aalborg and La Voulzie case studies, the agricultural N surplus pressure indicator is identified and reconfirmed as a suitable indicator as it is the most significant, prevalent, effective, and easy to use indicator regarding nitrate contamination of water.
Measured nitrate leaching below the soil zone would be the most appropriate state indicator but is seldom collected because sampling equipment to measure leaching is very costly to install and to maintain for monitoring, and the results can be difficult to upscale. However, in this study nitrate leaching data from pore water were available from Tunø, Denmark. This is an exceptional case and here we show how they can be used in combination with the N surplus and groundwater nitrate data. In general, the more abundant state indicator such as nitrate concentrations in groundwater is recommended as this is the standard state quality indicator.
However, it is important to adjust the choice of the nitrogen indicators to the purpose and scale of the study.
- For the evaluation of the effect of mitigation measure at the farm scale, the nitrate concentrations of soil pore water right below the topsoil (rooting zone) is recommended. This indicator has the shortest lag time; therefore, the effect of the implemented mitigation measure can be seen almost immediately (within 1 year). However, it only represents the condition at the very locally collection point. Thus, monitoring at multiple points in the area is recommended.
- For the evaluation of the mitigation measures at the catchment scale, the nitrate concentration in oxic groundwater is recommended. In nitrate-reducing or reduced groundwater, nitrate concentrations will be affected by the natural processes, and therefore, the effect of mitigation measures cannot be clearly seen. The groundwater chemistry integrates the effects within its recharging area; however, groundwater should be monitored at multiple locations to increase the representativeness of the data.
- The annual average concentration of nitrate in groundwater is recommended to monitor the status of the water quality. Both the groundwater and drinking water standards for nitrate are 50 mg/L. The sampling frequency per year may vary depending on the hydrogeological settings and lag time. If the lag time is relative long, then a sampling frequency of once every year or up to every 2-3 years may be acceptable. On the other hand, relatively short lag times will require more frequent sampling as in the case study of Aalborg in Denmark where there is preferential flow in macro pores.
2. Pesticide indicators
Selecting directly appropriate pesticide indicators is much more difficult than for nitrogen due to the lack of long time series of both pesticide application amounts (pressure) and pesticides concentrations in water (state).
In the specific case of La Voulzie, the analyses of the two other pressure indicators (area of main crop type and amount of application of pesticides) regarding pesticide contamination of groundwater were appropriate choices of indicators. These indicators are transparent and easy to use and communicate to stakeholders. However, they cannot be abundant indicators because it is rare that a single pesticide product is used on all the agricultural fields having the same crop type in a catchment. Therefore, in some specific catchments with monotype conventional agriculture these two pressure indicators (area of main crop type and amount of application of pesticides) could be indicators of potential pesticide contamination.
In the case of lack of direct appropriate pesticide pressure data, an attempt can be made by using N surplus as the pressure indicator of intensive agriculture and probable use of pesticides as demonstrated in La Voulzie study site of France. In La Voulzie, the lag time analysis showed statistically significant results. However, the statistical significance does not necessarily indicate scientific robustness of these estimates. In La Voulzie, the fertilizer reduction program and atrazine ban were implemented in the last two decades. In this case the N surplus indicator could be used also as a pesticide pressure indicator because nitrate and atrazine follow a similar trend during the intensification of agriculture.
3. Link indicators
At the two Danish sites, the lag times of nitrate estimated using the CCF analysis were comparable to the water ages estimated using environmental tracers (CFCs), but in general the lag times were shorter than the water ages. For instance, for Island Tunø, the lag times estimated based on the CCF analysis were 5-11 years shorter than the water age measured using environmental tracer, CFCs. The difference might be small, but it may provide a valuable insight into the mode of contaminant transport.
The lag time may represent the shortest travel time that delivers the agricultural signal to the water sample collection point (advective flow only). In contrast, the groundwater age represents the mean residence time of the existing groundwater at the collection point. It is well known that groundwater even at a narrow sampling interval is a mixture of a wide range of ages (Weissmann et al 2002, Gooddy et al 2006). These data imply that water can be contaminated rapidly with a long residual contamination, and thus it may take a longer time to remediate it. The lag time mainly represents the signal propagation through the fast route while the groundwater age represents the average of both fast and slow route.
In La Voulzie, the statistically significant lag times were comparable for nitrate and pesticides using the N surplus pressure indicator. The estimated lag times for nitrate were 14 and 24 years, and for pesticides the values were 15 and 20 years for the main and bottom spring respectively while for the top spring the lag times were higher for pesticides (20 yr) than nitrate (8 yr).
Note: For full references to papers quoted in this article see