Interpretasi data geolistrik untuk penentuan batas antara air tanah tawar dan payau di daerah Dalen, Provinsi Drenthe, Belanda

Abstract

ABSTRAK
Daerah penelitian terletak di Dalen, Provinsi Drenthe, Belanda. Di area ini terdapat sumur produksi air bersih yang telah berproduksi sejak awal abad 20 dan air yang dihasilkan tetap bersifat tawar. Padahal menurut studi TNO (1978), di selatan Dalen, yaitu di Coevorden, batas antara air tanah tawar dan payau (interface) pada tahun 1976 berada pada kedalaman sekitar 20 m di bawah permukaan tanah. Penelitian bertujuan untuk mengetahui posisi interface dengan menggunakan data geolistrik. Data geolistrik diinterpretasi untuk mengetahui posisi batas litologi beserta nilai resistivitas batuan dengan acuan data litologi dari sumur bor serta data resistivitas yang diperoleh dari loging. Kemudian berdasarkan pada data kualitas air tanah, nilai konduktivitas batuan dikaitkan dengan nilai kadar klorida, sehingga didapat posisi kedalaman interface. Interface ini terletak pada kadar klorida 150 mg/L, sesuai dengan standar air minum Belanda. Pola posisi kedalaman interface yang diperoleh dari studi ini pada dasarnya sama dengan hasil studi TNO (1978). Interface terdangkal berada pada permukaan tanah, terletak di selatan Coevorden, di area antara Sungai Vecht dan Schoonebeker Diep. Dangkalnya interface tidak terkait dengan keberadaan sumur produksi di daerah ini. Sehingga sumur-sumur tersebut, termasuk yang terdapat di Dalen, masih aman dari kemungkinan terjadinya upconing air payau.
Kata kunci: geolistrik, kadar klorida, interface, air tanah payau

ABSTRACT
The study area is located in Dalen, Drenthe Province, The Netherlands. In this area, there is a fresh water producing well that has been in production since the early 20th century. Meanwhile, based on TNO study (1978), to the south of Dalen, namely in Coevorden, the boundary between fresh and brackish groundwater (interface) in 1976 was at a depth of approximately 20 m below the ground surface. The purpose of this research is to determine the interface position using geoelectrical data. The geoelectrical data is interpreted to determine the depth of each lithology and its resistivity using borehole data and loging data as the reference. Furthermore, based on the water quality data, a relationship between chloride concentration and electrical conductivity values is established. Based on this relationship, interface positions can be determined. The interface is located at groundwater with chloride concentration of 150 mg/L, according to the drinking water standard in The Netherlands. The interface positions obtained from this study has a similar pattern with the result of TNO’s research (1978). The shallowest interface is at the surface, located in Coevorden, between Vecht River and Schoonebeker Diep. The interface positions are not connected with the production wells operating in the study area. So that, those wells are still safe from the upconing brackish water.

Keywords: geoelectrical, chloride concentration, interface, brackish groundwater