Jurga Arustiene, Lithuanian Geological Survey Assessment of chemical status in fresh/salt groundwater transition zones - Lithuania Jurga Arustiene, Lithuanian Geological Survey

Groundwater bodies Upper Devonian Stipinai (1), Kėdainiai-Dotnuva (2) and Suvalkija (3) located in central part of Nemunas RBD Main reason for delineation of these groundwater bodies is fresh/salt groundwater transition zones, which naturally occurs in aquifers used for drinking water supply. The historical data indicates that groundwater abstraction in fresh/salt water transition zones is risky because it can disturb the natural equilibrium existing in not-pumped aquifers and could deteriorate groundwater quality

The abnormally high concentrations of two problematic indicators - sulphates and chlorides, exceeding TV’s set in drinking water standard, are observed in part of the wellfields: Upper Devonian Stipinai (1) – 24%, Kėdainiai- Dotnuva (2) – 42% and Suvalkija (3) – 76%. Chemical and quantitative status of these groundwater bodies was assessed as good, but they were identified as groundwater bodies potentially at risk. As there was lack of data in order to access trends in groundwater quality, program of measures foresaw operational monitoring of problematic quality indicators in all wellfields.

Pressures In Nemunas river district 3 groundwater bodies were identified as potentially at risk due to same reason – occurrence of fresh/salt groundwater transition zones and possible risk of groundwater quality deterioration due to groundwater abstraction -  but situation in each of groundwater body is different and has to be assessed separately. Taking into account natural background levels of sulphates and chlorides TVs for chemical status assessment were set higher than drinking water standard - 500 mg/l and 350 mg/l.

Upper Devonian Stipinai groundwater body Groundwater body is comprised from Quaternary, Permian and upper Devonian aquifers. Upper Devonian Stipinai aquifer is main aquifer used for drinking water supply. Current groundwater abstraction rate is low, makes 14% from available groundwater resources. Due to irregular distribution of gypsum layers within this aquifer high concentrations of sulphates are observed in part of wellfields (24%)

Historical monitoring data from one of the largest wellfield (Bubiai) located within groundwater body, indicate impact of groundwater abstraction to groundwater quality. During the period of the most intensive groundwater abstraction average concentration of sulphates increased from 120 mg/l in 1970 to 310 mg/l in 1999, currently, due to groundwater abstraction decrease, concentrations of sulphates are stable ~ 280 mg/l. Same situation – stable concentrations of sulphates - is observed within entire groundwater body.

Kėdainiai-Dotnuva groundwater body was delineated in the periphery of Upper-Middle Devonian aquifer system. Groundwater body is comprised of 2 different aquifers of Upper Devonian and main productive aquifer of Upper-Middle Devonian. Only the upper part of this thick groundwater system contains fresh water.

Groundwater abstraction rate from this groundwater body is moderate - makes 27% from available groundwater resources. Special evaluation was made for a biggest wellfield of Kedainiai. Estimated salt water input into the budget from below is 16–23 %, and from above, 13–20 %. These inputs regulate Cl and SO4 trends. Production wells, installed in the middle part of this aquifer system already provide groundwater with concentrations of Cl and SO4 close to TV values. Groundwater quality in the upper part of the exploited aquifer system was and is good and resources are enough to ensure drinking water need for a nearest future.

Suvalkija GWB Groundwater body Suvalkija was delineated in the part of Upper-Middle Cretaceous system, where anomalies of chlorides occur. In the areas of faulty zones, the most often in river valleys, salt water ascends from deeper aquifers, resulting high concentration of chlorides. Wellfields, where average concentration of chlorides exceeds drinking water standard (250 mg/l) makes 76 %. As current groundwater abstraction rate is low – 12 % from available groundwater resources, monitoring data indicates some decrease of chlorides.

Measures As there was lack of data in order to access trends in groundwater quality, program of measures foresaw operational monitoring of problematic quality indicators in all wellfields, extracting more than 10 m3/day located in groundwater bodies at risk and now is carried out from 2013 and is foreseen till the end of the 2nd Nemunas RBD management plan. Currently, with no evidence of groundwater quality deterioration within entire groundwater bodies at risk, groundwater quality in wellfields is considered as drinking water supply issue, even if increase of sulphates and/or chlorides are observed.

Justification for applying Article 4(4) or 4(5) WFD Articles 4(4) or 4(5) are not currently applied, but if groundwater abstraction will increase in the future it could be case. Due to complicated hydrogeological conditions of wellfields located close or within fresh/salt groundwater transition zones prevention of groundwater quality from deterioration could be challenging.

Conclusions and lessons learned The five-year period of groundwater monitoring was not enough to decide if risk of groundwater quality deterioration due to groundwater abstraction exists. One of the reason is current groundwater abstraction rate, which makes only 12-27% of available groundwater resources. Each groundwater body due to different hydrogeological and economic conditions is unique and should be assessed separately. Dual standards are applied to the wellfields in fresh/salt groundwater transition zones, because TV’s of sulphates and chlorides were set based on NBL’s and are higher than TV’s for drinking water standard. This makes confusion when representing data to public. Currently going on projects of drinking water infrastructure development not always count natural hydrogeological conditions.