Gypsum and the groundwater directive

The European water policy aims at:

­– reducing the impact of human activity on the freshwater quality;

­– where necessary, progressively bringing the water back to a good status, which depends on its use (drinking, crop, industrial …);

– promoting a sustainable water use.

The monitoring and reporting must be operational by December 2012, but the threshold values for groundwater quality monitoring must have been established by the member states by 22.12. 2008.

Sulphate is a parameter to be taken into account in the derivation of threshold values by the member states when it is present in the groundwater body as the result of human activities. The concentration of sulphate in groundwater largely depends on the condition of the natural ground: in gypsum areas groundwater may be saturated (1.45g/l of sulphate) and in this case the addition of gypsum will not change the sulphate concentration.

­– Groundwater values of up to 2950 mg of SO₄^2- per litre are reported in Belgium and up to 4300 mg SO₄^2- per litre in France.

– In South-East Lower Saxony, a maximum value of 1455 mg SO₄^2- per litre is found, while 28.5  % of the drinking-water production installations show sulphate concentrations of over 250 mg/l.

­– In Thuringia, maximum values of up to 2496 mg SO₄^2- per litre (Muschelkalk) are reported, as well as an arithmetical average of 683mg SO₄^2- per litre (Keuper).

The freshwater issues are mainly covered by 4 directives:

– The Water Framework Directive (WFD) adopted on 23.10.2000 [1], aims to establish a framework for the protection of inland surface waters, transitional waters², coastal waters and groundwater³.

and 3 so-called “daughter directives”:

– The Groundwater Directive (GWD), adopted on 12.12.2006 [2].

­– The Environmental Quality Standards (EQS) Directive for surface waters, was adopted on 16.12.2008 [3].

– The Drinking Water Directive (DWD), adopted in 1998 [4], soon to be revised.

The content of the directives is indicated in Figure 1: Surface waters feed the groundwater bodies, and drinking water (as well as water for other uses, including industrial use) is abstracted from either surface or groundwater. This is why the three directives (4 with the framework directive) are closely linked and why all parameters listed in them should be addressed.

In order to help the member states in implementing the GWD, the commission decided to create guidelines on establishing threshold values. The first document was very complex and not fully accepted by the member states, so an interim version was approved in late 2007 as a working basis. Indeed, the threshold values had to be established by each member state by 22.12. 2008 and published in the River Basin Management Plan.

Main elements of the guidelines:

– The threshold values aim to define and, where necessary, to recover a good water status. Concentrations of substances or values of indicators are assessed at different monitoring points in the groundwater body, and, according to the status of water and the way the groundwater body is fed, limit values are then set for sources, e. g. a plant.

– The threshold values should be set at the most appropriate scale: national, river basin or groundwater body.

Most important: the TVs should not be established below the background levels (BL)⁴. The procedure is as follows (for each substance/indicator that may represent a risk for the groundwater)

– assessment of the Background Level (BL)

– establishment of the Criteria Value (CV). This value could be defined as an optimal value, taking account of

• the input: surface water intrusion

• the output: use of groundwater: drinking, crops, industry, others…

– threshold values (TV) setting:

• If CV ≤  BL then TV = BL

• If CV > BL then BL < TV ≤ CV

­– according to the targeted threshold value and the actual status of the groundwater, limit values are established, for the sources concerned with substances or parameters which cause the groundwater to be at risk, and included in permits, general binding rules or codes of practice

Although the Groundwater Directive addresses a limited number of substances and parameters, threshold values should be set for any substance or parameter which causes the groundwater to be at risk. Member states are also free to be more stringent than the guidelines, in accordance with national strategies and existing regulations, when setting the threshold values. Member states may remove a threshold value from the list when the groundwater body is no longer at risk.

The final text of the practical guidelines and technical specifications for the derivation of threshold values for the chemical status and trend assessment was officially approved by the Water Directors in November 2008 and maintains the elements above-mentioned.

Within the above-mentioned context, Eurogypsum developed a case study which is included in the annexes of the Guidance on Groundwater, status and trends. The case study was developed on the basis of a survey carried out by Ingenieurbüro Völker in December 2006 analysing the geological impact on background values of ground- and surface waters with sulphates in gypsum deposits [5].

Contact of water with gypsum deposits results in high sulphate concentrations. Values above 500 mg/l are indications for a direct contact of water with the gypsum bedrock in the ground. Because the sulphate content is caused by calcium sulphate (gypsum), higher values of water hardness can also be detected.

The conductivity is an additional helpful indicator in outside measurements to distinguish aquifers with or without gypsum contact. In this case the influence of other water constituents on the conductivity has to be taken into account, especially chloride. But, conductivity is a good indicator when chloride and other influences in the area to be investigated are evaluated as not relevant. In known gypsum deposit areas the author distinguishes between rain water infiltrated karstic pools and others that have contact to the sulphate bedrock. Typical values of conductivity, sulphate and hardness of water in a gypsum karst area of 0.6 km² in lower Saxony are summarized in Table 1.

But, in the same gypsum deposit area there are waters without contact to the gypsum bedrock. This is clearly detectable by the measurement of the indicators sulphate and conductivity (Table 2).

The discharge of the karst area into a creek influences following aquifers over long distances. The water itself has a high environmental and ecological quality status. The sulphate content has never been measured up to this study. The creek comes from a “Buntsandstein” area and shows only minor sulphate values (1962 mg/l/measuring period from 26.04.2002 to 20.03.2004) within a distance of several kilometres in a valley. When contact to a creek from a karst gypsum area is established, this situation changes significantly. The “sulphate free” creek suddenly changes into “sulphate-rich” water. The measurements at measuring point 44 vary from 91 mg/l to 472 mg/l sulphate.

The significance of the karst region increases particularly in high- temperature summer time when the original spring activity drops to a low value. The dissolved sulphate is transported over several kilometres into a gypsum-free area and can be clearly detected after a distance of 10 km from the geological source. All sulphate concentrations measured are only caused by a natural background situation without pollution effects. The influence of human activities can be excluded in all the above cases. The variations in the sulphate concentrations can be explained by dilution of groundwater or surface water through seasonal rain infiltration.

Groundwater aquifers that are in contact with gypsum and ­other ground (in this example granular soil) actually show the full sulphate saturation. But, whenever groundwater formation by infiltrating rainwater occurs, a short-term dilution concentration can be measured. A higher fluctuation is not relevant because the substance gypsum dissolves quickly. To demonstrate, an overview of the dilution of fresh gypsum in creek water is given in Figure 2.

Unpolluted water normally shows concentrations of less than 50 mg/l sulphate. However, this concentration will often be exceeded in gypsum karst regions. The sulphate concentration in those regions can normally be detected in a range between 500 mg/l and 1400 mg/l. Those sulphate concentrations result from the dissolving of natural gypsum. In those areas no acidic water is formed and the ecological damage for which sulphate acidic water is known is absent. In the gypsum karst areas, the pH value of the water is slightly alkaline (pH 7.2). The geological background sulphate does not show any toxic effects on the environment. The dissolving of sulphate from gypsum deposits is a hydro-geological effect that cannot be stopped by any measures.

Therefore, it is recommended that, if it is demonstrated that the sulphate is of natural origin only and it is proved that no anthropogenic input occurs, threshold values for sulphate do not need to be established by the member states. Where sulphates result from both origins (natural and anthropogenic), determination of a TV remains mandatory as long as it represents a risk of failing good status. In this case, investigating on cations (ex. Cu⁺⁺) could lead to a better understanding.

As a minimum requirement, chloride (to detect seawater intrusion) and pH (to detect sulphide oxidation from pyrite) have to be detected in addition to sulphate. It could also be further recommended that in countries with gypsum deposits or any other gypsum-containing rock-sequences, groundwater threshold values for sulphate should only be established on a local level and with the knowledge of all human activities.