Water quality analysis of Bembem Doline Pond in Gunungsewu Karst area, Gunungkidul regency

Abstract

Dry seasons often lead to drought in Panggang Subsystem, part of the undeveloped Gunungsewu Karst that has minimal water resources in its springs and underground rivers. The local people have relied on karst lakes as the primary water source for various activities, such as bathing and cloth washing. Bembem Doline Pond is located in Giriasih Village, Purwosari District, Gunungkidul Regency and is part of the Panggang Subsystem. Aside from its intensive use, many factors are contributing to its water quality degradation, namely seasons, doline pond characteristics (e.g., water volume), landuse on the watershed, and the pattern of water use. This research intended to identify (1) the characteristics and (2) the water quality of Bembem Doline Pond in the early and mid-rainy season. For the water quality analysis, samples were collected from five points, which were selected by purposive sampling technique. This sampling took into account the location of the water inflow and part of the doline pond where human activities took place in November 2018 and January 2019. The test included three types of water parameters, namely physical (temperature, EC, and TDS), chemical (pH, nitrate, phosphate, detergent, and BOD), and biological (total coliform). The results showed that the water volume of Bembem Doline Pond in the mid of rainy season is 1,467.4 m3. Landuse in the watershed are dominated by plantation (45%) and dry farmland (34%), and some were covered by water bodies (14%), roads (2%), buildings (4%), and settlements (1%). The water quality of the doline pond was found to depend on seasons and human activities on the doline pond considerably. The pollutant levels inclined to be higher in the early than the mid-rainy season. In the former, human activities mostly played a part as pollutant supplier, while in the latter, runoff water from agricultural fields in the watershed contributed the most. According to the Regulation of the Governor of the Special Region of Yogyakarta No. 20 of 2008, the majority of the test parameters exceeded the quality standards for Class II water.