Groundwater dependent terrestrial ecosystems (GWDTE) are wetlands which critically depend on groundwater flows and /or chemistries. As part of the assessment of groundwater status, we are required to assess if a GWDTE has been significantly damaged and if the pressure causing this damage has been transmitted via a groundwater body. The magnitude of damage (i.e. ‘significance’) is related to the societal (conservation in UK) importance of the features of the wetlands and the degree of change to these features resulting from the pressure.
Threshold values are needed to determine what groundwater concentrations of chemicals, if exceeded, would indicate a pressure that could be (or actually is) causing damage to the GWDTE. In practise the values are used as a risk screen to trigger further investigation where needed. A combination of a damaged GWDTE (such as failure to meet conservation objectives) and exceedance of the relevant threshold values triggers further investigations (see groundwater chapter of the UKTAG consultation document; UKTAG 2012). These investigations need to confirm whether damage has occurred, substantiate the hydrogeological and hydrochemical pathway(s) between the groundwater body and GWDTE, and the outcome would inform groundwater body status assessment.
This report explains how we have derived the GWDTE threshold values and sets out how the knowledge underpinning them can be improved in the future.
(Revised slightly July 2014)