LST LR1 Local retention potential — GIS support for small sewers

Local retention potential is a calculation of the risk of phosphorus reaching surface water within a DARO and is a weighting of hydrological and geological risk. The hydrological risk is calculated by relating topographical distance to water (the route the water is estimated to run, such as surface or groundwater, to the nearest permanent watercourse, distance), to the extent of the run-off at each individual point (where the water reaches the nearest watercourse, catchment area). The geological risk has been assumed to increase with reduced infiltration capacity (soil species) and thinner soil layers (soil depth) The assumption is that retention decreases if the water/wastewater cannot penetrate/infiltrated into the soil. High risk means low retention, i.e. less phosphorus is naturally fixed in the soil on the way to the receiving water. Low risk and higher retention results in a higher degree of fixing. The purpose of the GIS support is to create conditions at a national level for a reasonable, legally certain and more consistent assessment of the level of protection under Chapter 2, Section 3 of the Environmental Code when examining small sewers. The GIS support consists of maps showing the risk of impact on recipient/exposure point due to load from a drain. The maps have been produced through risk assessments based on the legislator’s instructions on criteria for assessing the high/higher level of protection for the entire surface of Sweden.