1. 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

2. 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.

3. 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

4. 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”

5. 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.

6. 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).

7. 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.

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

9. 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

10. 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 :
, , og

11. 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.

12. Trend reversal in 1980 Groundwater infiltration year/application year
Birgitte Hansen,
Trend reversal in 1980
Groundwater infiltration year/application year
ES&T: Hansen et al. (2011)

13. Nitrate in oxic groundwater
Birgitte Hansen,
Nitrate in oxic groundwater

14. Nitrate in oxic groundwater
Birgitte Hansen,
Nitrate in oxic groundwater

15. Nitrate in oxic groundwater
Birgitte Hansen,
Nitrate in oxic groundwater
5 year moving average

16. Nitrate in oxic groundwater
Birgitte Hansen,
Nitrate in oxic groundwater

17. Nitrate in oxic groundwater
Birgitte Hansen,
Nitrate in oxic groundwater

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

19. 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

20. 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

21. THANK YOU FOR YOUR ATTENTION
Lærke Thorling
Klaus Hinsby
THANK YOU FOR YOUR ATTENTION
Photo: Peter Klaus Warna-Moors