Submarine groundwater discharge (SGD) from tropical volcanic islands locally transports significant amounts of terrestrial nutrients into the coastal ocean, which can affect sensitive coastal ecosystems such as coral reefs. In northwestern Lombok, terrestrial submarine groundwater discharges through several submarine springs at the seafloor into a coral reef. In order to understand the transport mechanisms of nutrients from land into the reef, we investigated nutrient fluxes via SGD within the area using a combination of different methods, including recharge estimates in the coastal catchment area, echosounder profiling of the seafloor, discharge measurements, and hydrochemical analyses (stable isotopes of water, radon, and nutrients). Different types of springs can be distinguished based on their morphology. One spring showed a “crater”‐shaped structure while other springs consisted of fissures. Recharge rates in the coastal catchment amounted to 5 L s−1, while discharge rates from the largest spring were 5 L s−1 for total SGD and 2.5 L s−1 for fresh SGD, which is similar to discharge rates from smaller onshore springs at the slopes of a volcano. SGD was in general a source of nutrients such as dissolved silicon (~ 226 mol d−1), phosphate (0.61 mol d−1), or total dissolved nitrogen (13.4 mol d−1). Nitrate concentrations varied among the different springs, probably due to different sources in the hinterland, transformations (e.g., denitrification), or biological uptake during transport from land into the ocean. As nitrate can considerably affect coral reef health, our findings suggest that especially nitrate should be monitored around submarine springs in Indonesia.

Pulled by gravity, fresh groundwater will come in contact with seawater at the downstream end of its flow system. Most previous work has discussed the shape of the seawater/freshwater interface on the basis of Ghyben-Herzberg's law. The groundwater, however, will come out to the surface as coastal springs or predicted by the law, the freshwater also can flow out from the bottom of the sea. The event of freshwater flows from the bottom of the sea or submarine is named submarine groundwater discharge (SGD). The analysis shows that the freshwater flows discharges along the interface, even as a submarine springs. Submarine groundwater discharge (SGD) is now recognized as an important pathway between land and sea. The understanding of SGD will facilitate the development of water resources and the evaluation of environments. This paper reviews the scientific significance of SGD evidence in Indonesia. It is concluded that while the SGD process is essentially ubiquitous in Indonesia coastal areas, the assessment of its characteristic should be made base on geology and hydrogeological condition. SGD estimation has been made in 6 locations, Kata Kunci : Keluaran airtanah di lepas pantai, mata air, air tawar, pantai, Indonesia Lubis, Rachmat Fajar, et.al / Riset Geologi dan Pertambangan

ABSTRAK Kawasan pesisir Pademawu yang beralih fungsi menjadi pusat pertanian garam menimbulkan beberapa dampak negatif yang salah satunya adalah permasalahan kerentanan sumber daya air tanah. Beberapa sumur gali milik penduduk sudah berubah menjadi payau hingga asin. Evaluasi terhadap kualitas air tanah di kawasan tersebut sangat dibutuhkan untuk mengetahui kerentanan air tanah. Penilaian kerentanan air tanah dilakukan berdasarkan parameter hidrogeokimia dan indeks kualitas air Water Quality Index (WQI). Fasies hidrokimia air tanah didominasi oleh fasies CaHCO3 kemudian NaHCO3 dan NaCl. Nilai rasio Na/Cl dan Cl/HCO3 menunjukkan bahwa dalam air tanah daerah penelitian telah terjadi proses pencampuran air laut ke dalam akuifer dengan kategori penyusupan air laut sedikit hingga agak tinggi. Kondisi tersebut didominasi oleh proses infiltrasi air tambak garam ke dalam akuifer. Nilai Water Quality Index (WQI) berkisar 46,69-736,42, kategori WQI good water mendominasi wilayah penelitian sebanyak 45,45%, excellent 27,28%, poor water 18,18% dan satu sampel air masuk kategori 9,09%. Lokasi dengan kategori poor water dan very poor water berada di sumur gali penduduk yang berdekatan dengan garis pantai dan tambak garam. Pengaturan tataguna lahan antara kawasan pemukiman dan tambak garam sangat dibutuhkan sehingga tidak memperluas area kerentanan air tanah di pesisir Pademawu.

Geochemical evaluation of the species and potential sources of P in groundwater in and around Mataram City, Lombok Island, Indonesia can aid in the initial research on evaluating the fate of P when detailed geological information is unavailable. The results of ion chromatography and inductively coupled plasma-optical emission spectroscopy show that the concentrations of PO43– and total dissolved P (TDP) in groundwater range from approximately 0.1 to 8.5 mg l–1 and from 0.04 to 2.95 mg l–1, respectively. Dissolved inorganic P accounted for 86%, on average, of the TDP concentration, and PO43– represented the predominant P species in this groundwater. The potential sources for most of this dissolved PO43–, according to indices developed based on the PHREEQC software and groundwater quality data, could be the dissolution of hydroxyapatite and/or vivianite minerals. However, the potential sources of dissolved PO43– in groundwater with a TDP concentration of ≥ 1 mg l–1 is likely to be the reduction of Fe(III)–(hydro)oxides, the initial decomposition of organic matter, or the dissolution of carbonate-rich fluorapatite considering the Fe2+, dissolved organic carbon, Ca2+ and F− concentrations measured. In addition, as several groundwater samples had a TDP concentration of ≤ 1 mg l–1 and comparatively high concentration of NO3–, other potential sources of dissolved PO43– in this groundwater could be anthropogenic.

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