Studi Potensi Airtanah pada Cekungan Airtanah (CAT) Banyumudal, Kabupaten Kebumen, Jawa Tengah

Abstract

Cekungan Airtanah (CAT) Banyumudal terletak di Kabupaten Kebumen, seluas 67 km². Mengacu pada peta geologi regional, daerah ini tersusun atas Formasi Halang, Kalipucang, dan Gabon. Maksud penelitian ini adalah menganalisis potensi airtanah CAT Banyumudal, baik secara kualitas maupun kuantitasnya. Metodologi yang digunakan adalah pengumpulan data sekunder, pemetaan hidrogeologi, akuisisi geolistrik konfigurasi Schlumberger dan uji pemompaan. Pemetaan hidrogeologi dilaksanakan pada 153 titik minatan, meliputi 110 sumur gali, 20 sumur bor, dan 23 mata air. Hasil pengujian laboratorium menunjukkan nilai daya hantar listrik airtanah sebesar 115 - 1.251 μS/cm pada sumur gali, 507- 981 μS/cm pada sumur bor dan 259 - 604 μS/cm pada mata air. Derajat keasaman (pH) airtanah pada sumur gali sebesar 5,761 - 8,5, pada  sumur bor 7,013 - 8,333 dan pada mata air 6,747 - 8,204.  Hasil plot diagram Stiff menunjukkan adanya 12 tipe airtanah, sedangkan plot diagram Piper menunjukkan 4 fasies airtanah, yaitu fasies alkaline earth water predominantly hydrogencarbonate, fasies alkaline earth water with higher alkaline content predominantly hydrogencarbonate, fasies alkaline water predominantly hydrogencarbonate, dan fasies alkaline water predominantly sulphate chloride. Hal tersebut menunjukkan bahwa airtanah di daerah penelitian berkualitas sedang – sangat baik. Hasil interpretasi geolistrik menunjukkan bahwa litologi bawah permukaan berupa batulempung, batulempung pasiran, batupasir, batugamping, breksi dan tuf. Hanya akuifer bebas yang berkembang di daerah penelitian. Cadangan statis airtanah di daerah penelitian sebesar 200,7×10⁶ m³ sedangkan cadangan dinamis sebesar 33,4 × 10⁶ m³. Daerah penelitian dapat dibagi menjadi dua zona potensi airtanah, yaitu zona potensi airtanah rendah pada akuifer bebas dengan debit optimum 0,553 -  1,492 L/dtk dan zona potensi airtanah sedang pada akuifer bebas dengan debit optimum 2,651 -  3,024 L/dtk.

ABSTRACT

Banyumudal Groundwater Basin (Banyumudal GWB) is located in Kebumen Regency, covering 67 km² area. According to the regional geological map, the study area consists of Halang, Kalipucang and Gabon Formations. This study aims to analyze groundwater potency in Banyumudal GWB in quality and quantity. The method of study includes secondary data collection, hydrogeological mapping, geoelectric acquisition with Schlumberger configuration and pumping test. Hydrogeological mapping was done from 153 locations, consist of 110 dug wells, 20 deep wells and 23 springs. Result of the laboratory test shows that groundwater has electrical conductivity (EC) between 115 – 1,251 μS/cm in dug wells, 507 - 981 μS/cm in deep wells and 259 - 604 μS/cm in springs. Groundwater shows an acidity (pH) of 5,761 - 8,5 in dug wells, 7,013 - 8,333 in deep wells and 6,747-8,204 in springs. Result of the Stiff diagram plot shows that there are 12 types of groundwater, while the Piper diagram plot shows 4 groundwater facies namely alkaline earth water predominantly hydrogencarbonate, alkaline earth water with higher alkaline content predominantly hydrogencarbonate, alkaline water predominantly hydrogencarbonate, and alkaline water predominantly sulphate chloride facies. This result generally represent that groundwater in the study area is in the very good – medium range quality. Geoelectric results indicate subsurface litology as claystone, sandy claystone, sandstone, limestone, breccia and tuff.  It seems that only unconfined aquifers which exist in the study area. Groundwater reserves in study area include static reserves in amount of 200,7×10⁶ m³ and dynamic reserves in amount of 33,4 × 10⁶ m³. Based on quality and quantity of groundwater, there are two groundwater potential zones, that are low groundwater potential zone in unconfined aquifer with optimum discharge of  0,553 – 1,492 L/s and medium groundwater potential zone in unconfined aquifer with optimum discharge of 2,651 -  3,024 L/s.