Lõuna-Eesti ja Põhja-Läti Kesk-Alam-Devoni põhjaveekompleksi põhjavee päritolu ja selle keemilise koostise kujunemine [Bakalaureusetöö. Juhendaja: J. Pärn; kaasjuhendaja: R. Vaikmäe]

Bakalaureusetöös uuritakse ja analüüsitakse varem avaldatud andmestiku põhjal (kokku 106 analüüsi) Kesk-Alam-Devoni veekompleksis esinevaid põhjaveetüüpe, nende päritolu ja levikut ning kujunemist. Nende tunnuste põhjal eristatakse Kesk-Alam-Devoni põhjaveekompleksis aktiivse ja aeglase veevahetuse vööndid. Bakalaureusetöö uurimispiirkonnaks on Lõuna-Eesti ja Põhja-Läti Kesk-Alam-Devoni põhjaveekompleks.

Põhjaveetüüpide väljaeraldamiseks kasutatakse Pieter Jan Stuyfzandi väljatöötatud meetodit, mis kirjeldab lisaks põhjavees esinevatele peamistele katioonidele ja anioonidele ka vee mineraalsust, leelisust ja katioonvahetuse indeksit. Esmalt määrati veeproovide valimis olevale geokeemilisele andmestikule tuginedes Kesk-Alam-Devoni põhjaveekompleksis esinevad üldisemad veetüübid vee peamiste katioonide ja anioonide alusel. Stuyfzandi meetodiga eraldati seejärel välja detailsed veetüübid, mille põhjal uuriti täpsemalt põhjavee keemilise koostise kujunemist.

Aktiivse ja aeglase veevahetuse vööndi eristamiseks kasutati uuringus lisaks põhjavee tüüpidele ka olemasolevaid põhjavee isotoopkoostise andmeid. Sellest tulenevalt kirjeldatakse erinevate veevahetuse vööndite esinemist Kesk-Alam-Devoni põhjaveekompleksi majandamiseks loodud põhjaveekogumites.

Abstract

Origin and chemical composition of groundwater in the Lower-Middle Devonian aquifer system in southern Estonia and northern Latvia
The thesis studied and analysed the chemical types of groundwater in Lower-Middle Devonian aquifer system. The types of groundwater were found by using available data from previous studies that provide necessary geochemical information. The dataset consisted of a total of 106 water samples. The data was used to delineate the main water types and characterize their origin and spatial distribution. Furthermore, the extent of active and slow water exchange zones was investigated. The latter is an important feature to assess the sustainability of groundwater abstraction from the aquifer system. The study area comprises the Lower-Middle Devonian groundwater system in southern Estonia and northern Latvia.

The investigation of water types is done by using a method developed by Pieter Jan Stuyfzand (Stuyfzand 1986, 1989, 1993). The method uses and provides information about four important aspects of groundwater quality. The aspects are the following: groundwater salinity, hardness/alkalinity, the most abundant cations and anions, and the cation exchange index also known as BEX. By using the method general groundwater types (main cations and anions) and detailed water types (Stuyfzand method) were delineated.

In the course of the research, the distribution of δ18O values in groundwater was also investigated. The δ18O isotopic values can help to determine, whether a water sample is located in the zone of active or slow water exchange and also gives indirect information about the groundwater age. Data from 37 previously published isotopic analyses was used in this analysis. The δ18O value of -14‰ was used as a threshold to distinguish between active and slow water exchange zones as suggested by Pärn (2018).

The results of the analyzis showed that five main groundwater types occur in the Lower-Middle Devonian aquifer system: Ca-HCO3, Mg-HCO3, Na-HCO3, Ca-SO4 and Na-Cl. The Ca-HCO3 and Ca-SO4 water types were mainly found in and around the uplands of Southern Estonia and the outcrop of the aquifer system. Ca-HCO3 type groundwater was fresh with high alkalinity and was not affected by cation exchange processes (water types of G3Ca-HCO30 või g3Ca-HCO30).

The Mg-HCO3 water type was found in the northeastern part of the aquifer system and in the southern part of the aquifer system further away from the main recharge areas. This water type was affected by cation exchange (water type g3Mg-HCO3+). The Na-HCO3 water type occured in the coastal areas of southwestern Estonia and inland around ancient buried valleys. Na-HCO3 water type had average alkalinity and was affected by cation exchange of the freshening type that occurs when fresh recently formed groundwater flows into the aquifer previously containing groundwater with higher salinity (water type F2Na-HCO3+).

The Na-Cl water type occurs in the islands of Kihnu and Ruhnu in the Gulf of Riga. Na-Cl type groundwater had high salinity, low alkalinity and negative cation exchange index (water types B2Na-Cl- and f2Na-Cl-). Na-Cl type groundwater with even higher salinity was found in central Latvia (water types b1Na-Cl- and b0Na-Cl-). Negative cation exchange index in both cases refers to salination type cation exchange, which is caused by intrusion of the saltwater into the aquifer previously containing fresh groundwater.

The δ18O isotopic values clearly showed the occurrence of older groundwater of glacial origin in the islands in the Gulf of Riga and in the northeastern part of the aquifer system in the Tartu County. The groundwater in the central Latvia is also probably very old and those three areas represent the slow water exchange zone in the aquifer system. The transition from slow to active water exchange zone occurs in western mainland part of the Lower-Middle Devonian aquifer system and in the eastern part aquifer system in areas that are located farther away from the main recharge areas. The active water exchange zone occurs in and around the South-Estonian uplands and in the outcrop area of the Lower-Middle Devonian aquifer system.