Karakterisasi perairan Danau Toba dengan Isotop alam

MANURUNG, Simon, Promotor Prof.Dr.Ir. Prayoto, M.Sc

2005 | Disertasi | S3 MIPA (Ilmu Fisika)

Penelitian tentang karakter Perairan danau toba telah dilakukan untuk mengungkapkan faktor-faktor penyebab gangguan terhadap fluktuasi permukaan air Danau Toba. Dilakukan pendekatan dengan metode isotop alam stabil Oksigen-18, Deuterium serta isotop radioaktif Tritium. Sejumlah sampel air berupa air hujan, mata air, air danau dari kedalaman 0,5; 25, 50, 75, dan 100 m dikumpulkan dan dianalisis kandungan isotopnya. Hasil analisis isotopnya diplot terhadap garis meteorik lokal dan global serta terhadap kedalaman untuk mendapatkan pola penyebaran kandungan isotop tersebut. Variasi dan distribusi kandungan isotop memperlihatkan perbedaan yang signifikan sebagai pengaruh dari musim, namun tidak diindikasikan adanya stratifikasi hingga kedalaman 100 m, menunjukkan bahwa terjadi proses percampuran yang merata dari air permukaan hingga kedalaman 100 m. Berdasarkan parameter keseimbangan isotop air danau tidak terlihat adanya perbedaan signifikan dari pengaruh proses alam terhadap gangguan fluktuasi permukaan air Danau Toba. Gangguan keseimbangan lebih disebabkan oleh pemanfaatan air danau yang melampaui debit aliran masuk (anthropogenic uses). Tidak terbukti adanya kebocoran air danau berdasarkan hasil analisis kandungan isotop dua mata air besar yaitu : Krasaan dan Manigom, masing-masing dengan kandungan isotop δ18O = -8,23 0/00 dan δD = -49 0/00 untuk Krasaan, δ18O = -8,15 0/00 dan δD = -46 0/00 untuk Manigom. Pengukuran sampel air sebuah sumur bor berjarak lebih kurang 50 m dari pinggiran danau dengan debit air 15 L/s adalah δ18O = -10,2 0/00 dan δD = -63,8 0/00 menunjukkan daerah asal air pada elevasi antara 1100 sampai 1300 m dpl. Kandungan isotop kedua mata air menunjukkan finger print yang berbeda dengan kisaran kandungan isotop danau antara –5,1 0/00 dan –4,5 0/00 untuk δ18O, dan antara –42,5 0/00 dan –30,1 0/00 untuk δD. Dengan mengambil simpangan baku kesalahan penelitian 0,1 0/00 untuk δ18O dan 1 0/00 untuk δD, maka dapat disimpulkan bahwa kedua mata air tersebut bukan hasil kebocoran dari Danau Toba. Hasil analisis kandungan Tritium dari dua mata air menunjukan umur 21,9 dan 29,04 tahun dan air Danau Toba menunjukkan umur 36,44 tahun. Angka ini menunjukkan relasi air Danau Toba dengan air tanah/ mata air sebagai hubungan aliran masuk. Hasil analisis isotop alam stabil Oksigen-18, Deuterium dan isotop alam radioaktif Tritium membuktikan arti teknik isotop lingkungan yang dapat memberikan kontribusi signifikan terhadap gangguan keseimbangan fluktuasi air Danau Toba sebagai akibat dari campur tangan manusia terhadap Danau Toba yaitu pemanfaatan air keluaran yang melampaui aliran masukan sehingga menimbulkan gangguan keseimbangan fluktuasi alami. Hasil analisis isotop alam ini juga telah dapat mengidentifikasi daerah masukan air (recharge area), informasi yang sangat penting untuk manajemen sumber daya air.

Investigation on characterization of Lake Toba was carried out to determine some factors which cause the disturbance of Lake Toba’s water level fluctuation. The research was conducted using environmental stable isotopes Oxygen-18 and Deuterium as well as Tritium radioactive isotope. Several water samples, namely water from precipitation, springs, and lake at different depths (0.5, 25, 50, 75, and 100m), were collected. Isotopic compositions of these samples were analyzed using the mass spectrometer. Results obtained were plotted in terms of local as well global meteoric water lines and depth. Variation and distribution of the isotopic composition reveal a significant difference between dry and rainy seasons. However, the results do not indicate the a presence of stratification at the depth up to 100 m deep. This also means that a good mixing process occurred within the body of water from surface down to 100 m deep. Based on the isotopic balance of the lake water, it seems that the natural event does not significantly influence the water level fluctuation in Lake Toba. Based on the research, it is presumed that the cause of water level fluctuation in Lake Toba is due to the excessive amount of water outflow compared to the water inflow from Asahan River (anthropogenic uses). Isotopic compositions of two big springs, i.e. Manigom and Krasaan were analyzed to show the presence of leakage. It was thought that these two springs were formed as a result of leakage from Lake Toba. Isotopic composition of water from Manigom is -8.15o/oo and 46o/oo for Oxygen-18 and Deuterium, respectively. Meanwhile, the isotopic composition of water from Krasaan is -8.23o/oo and -49o/oo for Oxygen-18 and Deuterium, respectively. These values are very different compared to isotopic composition from the lake water. This means that the springs were not formed as a result of leakage from Lake Toba. Measurement of water sample from a deep well (75 m) 50 m from the lake shoreline with water flow rate of 15 L/s was completed. The isotopic composition for this sample is -10.2o/oo and - 63.8o/oo for δ18O and δD, respectively. These values are also very different from the average values the lake water which are between 5.1o/oo and -4.5o/oo for δ18O and between -42.5o/oo and -38.1o/oo for δD. The recharge area of the well is accounted to be between 1100 and 1300 m above sea level. Analysis of Tritium content of another two springs indicates their ages which are approximately 21.9 and 29.04 year old. Meanwhile, the Lake Toba water is estimated to be 36.44 year old. These figures indicate a close relationship between groundwater the lake water. The results from isotopic analysis indicate a normal inter-relationship among water variables which proves the significance of isotopic technique in the investigation of water level fluctuation in Lake Toba. The results from these environmental isotopes also identify the presence of recharge area which is very important for the management of water resource.

Kata Kunci : Ilmu Fisika,Hidrologi Isotop,Fluktuasi Permukaan Air,Isotop Alam