Natural Background Levels for Barium and Phosphorus in Groundwater. Udarbejdet af GEUS for Miljøstyrelsen, 2022

The work reported here was done as part of the project task defined by the Miljøstyrelsen “Opgavebeskrivelse – Fastsættelse af naturlige baggrundsværdier for fosfor og barium i de danske grundvandsforekomster” (see Appendix 1). The purpose was to propose natural background levels (NBLs) for barium (Ba) and total phosphorus (Ptot) for Danish groundwa-ters. NBLs for Ba and P have not been set previously in Denmark, so it was necessary to establish criteria for doing this, which required assessing what geological, geographical, and geochemical parameters would be relevant.

NBL is defined as a concentration level, for a chemical element or parameter in a body of groundwater, corresponding to no or very minor anthropogenic influence. NBLs are used for setting threshold values (TVs) in relation to the chemical status assessments of groundwater bodies in EU, as stipulated in the EU Water Framework Directive and the Groundwater Di-rective. Setting TVs and assessing the chemical status of groundwater bodies for Ba and Ptot is beyond the scope of this report.

The data used for calculating NBLs included all chemical analyses for Ba, Ptot, pH, and NO3- of groundwater samples from the period 2000–2021 for only the well-screens (called further “intakes”), which are part of the national groundwater monitoring program (GRUMO) or the waterworks wells used for drinking water production (BK). Data for pH and NO3- were in-cluded, as those were found to be important geochemical parameters based on an initial literature review. In addition, different meta-data for the intakes were also available, e.g. li-thology, groundwater body, location, etc. The dataset was quality assured and the values below the limit of detection were handled appropriately, se Chapter 1. The time-series were aggregated, so each intake is represented by a single value for the period. The aggregation was based on a mean calculated from the annual means for each element and intake. The data-processing procedures used here are similar those used in GEUS previous work on NBLs for trace metals for Vandplan III, as requested.

The final datasets used for the NBL calculation included 6558 intakes (6181 wells) for Ba and 8088 intakes (7348 wells) for Ptot. These intakes are associated respectively with 453 (22%) and 525 (26%) of the 2050 groundwater bodies in Denmark and cover the entire coun-try.

NBLs are calculated here as the 90th percentile of the aggregated data for each of the defined aquifer types with at least 50 intakes. The 90th percentile was rounded following specific rules that account for the relative analytical uncertainty of 15-20% for Ba and Ptot. Those rounded values are the NBLs for Ba and Ptot. The aquifer types were defined by combining the differ-ent classes for geology, location, redox and pH states (pH was only relevant for Ba):

• Location: 1) Jutland, 2) Funen, 3) Zealand, 4) Bornholm, and 5) the small islands close to Jutland
• Geology classes: 1) fractured carbonate rocks, 2) Quaternary sand, 3) pre-Quater-nary sand, 4) all geologically diverse units on Bornholm
• Redox classes: 1) reduced (NO3- ≤ 2 mg/l), oxic (NO3- > 2 mg/l)
• pH classes: 1) High (pH > 7), 2) low (pH ≤ 7)

NBLs were also calculated for the combination of only location and geology, to be used if there is no geochemical information for the groundwater samples from a specific groundwater body or for those cases when NBLs for specific geochemical condition could not be calcu-lated due to low number of intakes.

Six different NBLs were determined for Ba in the range 40–250 µg/l for the different aquifer types. All these were above the general quality requirement for fresh surface waters (EQS = 19 µg/l) and some are even above the maximum acceptable concentration (MAC = 145 µg/l). The lowest NBL for Ba is for the carbonate aquifers in Jutland (40 µg/l), while the highest is for the Quaternary sand with reduced conditions and high pH (250 µg/l). The Quaternary sand aquifers on Funen and Zealand are also with high NBLs (150 or 200 µg/l). These high NBLs could be to some extent be explained by the spatial extend of Tertiary clay layers in Denmark.

Eight different NBLs were determined for Ptot in the range 0.04–0.3 mg/l. Sand aquifers (both Quaternary and pre-Quaternary) have higher NBLs for Ptot than the carbonate aquifers, and
all reduced aquifers (NO3- ≤ 2 mg/l) have consistently higher NBLs than the oxic ones. Cur-rently there is no environmental, drinking water, or groundwater standard for Ptot. The aquifer types with NBLs exceeding the previous drinking water standard (0.15 mg/l) are: Quaternary and pre-Quaternary sand aquifers with reduced conditions, aquifers with reduced conditions on Bornholm, and the Quaternary sand aquifers on Læsø, Samsø, and Anholt. While car-bonate aquifers have low NBLs for Ptot in general, it should be noted that a relatively large area in North Zealand has consistently high Ptot concentrations in this type of aquifer. This should be considered when using the NBLs further.

Finally, it should be noted that phosphorus in surface water is regulated in relation to the annual total P transport, i.e. kg-P/year and not concentrations, i.e. mg/l as for groundwater.