KAJIAN KERENTANAN AIRTANAH TERHADAP PENCEMARAN DI DAERAH KARST GUNUNGSEWU (Studi di Daerah Aliran Sungai Bawah Tanah Bribin Kabupaten Gunungkidul dan Wonogiri)

MARGARETHA WIDYASTUTI, DRA.,MT., Prof. Dr. Sudarmadji, M.Eng.Sc.

2014 | Disertasi | S3 Ilmu Geografi

Airtanah karst merupakan salah satu sumberdaya alam yang potensial di kawasan karst Gunung Sewu, namun di sisi lain akuifer karst juga sangat rentan terhadap pencemaran. Penelitian ini dilakukan di Daerah Aliran Sungai Bawah Tanah Bribin yang menjadi sumber air utama untuk masyarakat khususnya untuk mendukung kebutuhan air di musim kemarau. Tujuan penelitian ini adalah 1) menganalisis distribusi spasial variabel kerentanan airtanah terhadap pencemaran (overlaying layer, konsentrasi aliran, curah hujan), 2) menganalisis distribusi spasial tingkat kerentanan airtanah terhadap pencemaran, dan melakukan validasi hasil kelas kerentanan airtanah terhadap pencemaran, 3) mengetahui faktor-faktor yang mempengaruhi tingkat kerentanan airtanah terhadap pencemaran, 4) mengevaluasi kesesuaian metode COP (Concetration of flow, Overlaying layer, Precipitation) untuk daerah kajian. Metode penelitian menggunakan metode survei, melalui pengukuran di lapangan secara langsung maupun survei data sekunder. Ponor, gua, dan dolin diidentifikasi melalui sensus, sedangkan pengambilan sampel tanah secara purposive berdasarkan unit seri tanah dengan mempertimbangkan formasi geologi dan bentuklahan. Proses pengolahan data mendasarkan metode COP, yang merupakan akronim C (concentration of flow/konsentrasi aliran), O (overlaying layer/lapisan pelindung) dan P (precipitation/curah hujan). Setiap variabel dan sub variabel mempunyai nilai di setiap kelas dan dihitung melalui operasi perkalian dan penjumlahan. Unit analisis yang digunakan adalah daerah imbuhan ponor dan daerah aliran sungai bawah tanah. Hasil kelas kerentanan airtanah terhadap pencemaran ditunjukkan dengan indeks COP, yang selanjutnya divalidasi menggunakan hidrograf aliran, khemograf dan uji perunut. Untuk mengetahui kesesuaian metode COP di daerah penelitian, variabel dari faktor C diuji statistik menggunakan analisis faktor. Hasil uji selanjutnya diterapkan untuk penilaian kerentanan airtanah terhadap pencemaran menggunakan metode COP modifikasi. Hasil penelitian menunjukkan distribusi spasial variabel kerentanan airtanah terhadap pencemaran bervariasi. Konsentrasi aliran (faktor C) sebagian besar menunjukkan kelas kerentanan sangat tinggi, sedangkan faktor O (Overlaying layer) dengan nilai proteksi sedang dan tinggi (kerentanan rendah dan sedang) dan P (Precipitation) sebagian besar menunjukkan nilai kerentanan rendah. Tingkat kerentanan airtanah terhadap pencemaran dominan terdapat pada kelas kerentanan tinggi dan sangat tinggi yaitu 63,04%. Hal tersebut dikuatkan dengan hasil validasi menggunakan hidrograf aliran, khemograf dan uji perunut yang menunjukkan respon aliran atau kualitas airtanah yang cepat terhadap imbuhan hujan atau zat perunut. Faktor yang berpengaruh terhadap tingginya tingkat kerentanan airtanah di daerah penelitian adalah konsentrasi aliran (faktor C). Kondisi ini dipengaruhi oleh banyaknya ponor yang terdapat di daerah penelitian sebagai tempat akumulasi aliran permukaan menuju sistem airtanah. Berdasarkan karakteristik daerah penelitian, variabel luas daerah imbuhan ponor dapat menggantikan variabel jarak terhadap ponor. Hal itu dapat dibuktikan dengan tingkat kerentanan airtanah di daerah penelitian menggunakan metode Gunungsewu lebih detil dibanding dengan metode COP sebelumnya. Metode Gunungsewu dapat diterapkan di daerah karst yang mempunyai keragaman kondisi basin tertutup. Kata kunci : kerentanan airtanah, pencemaran, metode COP, metode Gunungsewu

Karst groundwater is one of the potential natural resources in the Gunung Sewu karst area. On the other hand, karst aquifers are highly vulnerable to contamination. The research is conducted in catchment area of Bribin underground river that became the main source of water for the community, especially to support the water demand in the dry season. The purposes of this study are: 1) to analyze the spatial distribution of groundwater vulnerability to contamination variables (overlaying layer, the concentration of flow, rainfall), 2) to analyze the spatial distribution of groundwater vulnerability to contamination, and to validate the results of the groundwater vulnerability level to contamination, 3) to determine the factors that affect the vulnerability of groundwater level to contamination, 4) to evaluate the suitability of the COP method for the study area. The method of research was conducted by survey methods, through direct field measurements and survey of secondary data. Swallow holes, caves and doline were identified by census, while soils sampling were conducted by purposive sampling using soil series unit and consider the lithology formation and landform. The data processing method based on COP method, which is an acronym of C (concentration of flow), O (overlaying layer) and P (precipitation). Each variable and sub-variable has a value at each class, and then performed with the operation of multiplication and addition. The analysis unit used sinkhole recharge area and also underground river catchment area. The result of groundwater vulnerability to contamination class is indicated by COP index, which further were validated using hydrograph, chemograph and tracer tests. To determine the suitability of the COP method in the study area, the variables of C factor were tested statistically using factor analysis. The test results further were applied to assess the groundwater vulnerability to contamination using the COP method modification. The results show the spatial distribution of the variable of groundwater vulnerability to contamination varies. Concentration of flow (C) dominantly shows very high vulnerability values, whereas overlaying layer (O) include medium and high protection value (low and moderate vulnerability) and Precipitation (P) mostly show low vulnerability values. The level of vulnerability of groundwater to contamination leads to high and very high class which covered 63,04% areas. This is in line with the results of validation using a flow hydrograph, chemograph and tracer tests that show the quick response of groundwater quality, groundwater flow to the rainfall events or tracer substance. The factors that influences the level of groundwater vulnerability to contamination in the research area is concentration of flow (C). This condition is affected by by the number of sinkholes as point recharge of surface flow accumulation enters the groundwater system. Based on the condition of the study area, the sinkholes recharge area can replace the distance to the sinkholes variable. It can be proved by the level of groundwater vulnerability using Gunungsewu method give accurate result compared with previous COP method. Gunungsewu method can be applied in karst areas that have a diversity of enclosed basin conditions. Keywords: groundwater vulnerability, contamination, COP method, Gunungsewu method

Kata Kunci : kerentanan airtanah, pencemaran, metode COP, metode Gunungsewu; kerentanan airtanah, pencemaran, metode COP, metode Gunungsewu