Lærke Thorling and Klaus Hinsby Legacy and success of measures - groundwater dating as an important tool for assessment of trends and pollution history - a Danish experience Lærke Thorling and Klaus Hinsby

Articel 5, GWD Member States shall identify any significant and sustained upward trend in concentrations of pollutants, groups of pollutants or indicators of pollution found in bodies or groups of bodies of groundwater identified as being at risk and define the starting point for reversing that trend, in accordance with Annex IV. Member States shall, in accordance with Part B of Annex IV, reverse trends which present a significant risk of harm to the quality of aquatic ecosystems or terrestrial ecosystems, to human health, or to actual or potential legitimate uses of the water environment, through the programme of measures referred to in Article 11 of Directive 2000/60/EC, in order progressively to reduce pollution and prevent deterioration of groundwater.

The use of monitoring data Do we have a problem? WFD and GWD Is there an upcoming problem? Article 5.1 GWD Are we solving the problem? Article 5.2 GWD We always have to relate the problem to the drivers and responses

How to relate time series to drivers and responses? We cannot see the groundwater, we must make a model 5D understanding of the system: Every water sample represents a 5 D point: (x,y,z,tr,ts). (x,y,z) as the location of the filter, tr date of groundwater recharge, ts the date of sampling. This is easily stated – not so easily assesed, but The implications are huge and too often ignored Time series must be assessed in “relevant timescale”

What is groundwater age Groundwater age is the residence time of water in the subsurface at a given location. In a monitoring point or well, the age, is typically expressed as a mean residence or travel time (or a piston flow model age) – however the sampled groundwater is generally a mix of water with different ages The longer the screen wider is the age distribution of the samples e.g. the well screened in all three aquifers below will contain a mix of water types from less than a year to more than thousand years This has implications for the monitoring strategy, The interpretation of time series and the drawn conclusions.

How do we find the age of groundwater? Measure it: Applied tracers depend on the expected groundwater age - the most widely used and their dating intervals are shown below: Calculate it: Numerical groundwater flow models using synthetic particles (particle tracking).

Comparison of age distributions and mean residence times (PFM ages) in top and bottom of a drinking water well. Results from tracers and groundwater flow modelling 30 % of inflow Bconc: .043 Comparison of age distributions simulated by particle tracking groundwater model (PTM), tracer concentrations assuming binary mixing (BMM-DM-DM), and piston flow model ages (3H/3He, 85Kr and 39Ar PFM ages) in top and bottom of drinking water well with Bentazone concentrations (Bconc.) of 0.43 and <0.001 ug/L, respectively. The data strongly indicate that the Bentazone found in the drinking water well in the groundwater monitoring program enters the top of the drinking water well and that the Bentazone has recharged the aquifer after the introduction of Bentazone regulations. The example demonstrates the value of using multiple tracers and groundwater models for assessing groundwater age distributions especially when evaluating trends in monitoring wells with long screens where a high degree of mixing of water types with different flow paths and age occur. In shallow monitoring screens with short screens < 1m located in e.g. relative homogeneous sands close to a water divide the mixing of groundwater types and ages are rather limited 70 % of inflow Bconc: <.001 Jakobsen et al., in prep.

Trends of individual filters. (short monitoring filters) The good Trends of individual filters. (short monitoring filters) The ugly The real horror The bad

Nitrate in Danish groundwater We know we have a problem. GWB scale question: What are the trends (guid.18)? Answers are in real-time, all data National scale Question: Measures and their success? Answers in “residence time” only oxic groundwater

Success of measures and legacy Longer time series did not answer our questions. Better statistical methods did not answer our questions. Groundwater age estimates in the analysis of trends did Dating methods: CFC and 3H/3He in DK monitoring Dating results were used for analysis of nitrate trends, we want to do so systematically on a selection of pesticides too Subdivision of min. 8 years, in periods with similar regulation : 1940-75, 1975-85, 1985-1998 og 1998-2014

Birgitte Hansen, e-mail: bgh@geus.dk From samling year to infiltration year Example from one monitoring well: CFC-dating in 1998 Groundwater age (mean residence time) = 11 yr.

Trend reversal in 1980 Groundwater infiltration year/application year Birgitte Hansen, e-mail: bgh@geus.dk Trend reversal in 1980 Groundwater infiltration year/application year ES&T: Hansen et al. (2011)

Nitrate in oxic groundwater Birgitte Hansen, e-mail: bgh@geus.dk Nitrate in oxic groundwater 1944-2012

Nitrate in oxic groundwater Birgitte Hansen, e-mail: bgh@geus.dk Nitrate in oxic groundwater 1944-2012

Nitrate in oxic groundwater Birgitte Hansen, e-mail: bgh@geus.dk Nitrate in oxic groundwater 1944-2012 5 year moving average

Nitrate in oxic groundwater Birgitte Hansen, e-mail: bgh@geus.dk Nitrate in oxic groundwater 1944-2012

Nitrate in oxic groundwater Birgitte Hansen, e-mail: bgh@geus.dk Nitrate in oxic groundwater 1944-2012 2000-2012

Legacy and succes use of individual timeseries Only oxic series ES&T: Hansen et al. (2011)

Monitoring and reality check of effectiveness of measures Groundwater monitoring collects relevant data for water management. Special focus on establishment of time series for assessment of trends and chemical status according to WFD Monitoring always look back in time Models based on monitoring data can look into the future, make forecasts Groundwater age (residence time) is an important tool for linking and understanding monitoring and modelling results

Conclusions and recommendations from Danish assessments of trends and pollution history Trend assessment must be conducted in the right context - dating is the tool National action plans did improve the general groundwater quality, but…An additional challenge is to reach the objectives everywhere under all conditions An additional challenge is to reach the objectives everywhere under all conditions Common European tools and protocols for estimating and applying groundwater ages in trend assessment are highly recommended Possible proposal from the ERA-NET (GeoERA) on application of groundwater residence times in trend analysis

THANK YOU FOR YOUR ATTENTION Lærke Thorling lts@geus.dk Klaus Hinsby kh@geus.dk THANK YOU FOR YOUR ATTENTION Photo: Peter Klaus Warna-Moors