The phenomenon of rising groundwater level is an international issue found in several urban areas including Al-Ansab area of Oman. Water appears on the surface and shallower excavations and has affected houses and buildings, posing health and environmental threats and calling for intervention. Therefore, the main objective of this study is to identify the possible water leak sources that cause the rise in groundwater level using chemical, biological and isotopic analyses. In this study, 20 water samples were collected in April / May 2017 and March 2018 field visits, while 19 water samples were collected in December 2017. The samples were collected from different sources including groundwater, surface water, drinking water network and Haya wetland water. The physical parameters of water samples (pH, TDS, EC) were measured in the field using portable probes AP-5000. Water alkalinity was found through titration whereas major cations and anions were measured using ICP-OES and IC, respectively. In addition, biological parameters composed of total coliform and E.coli were analyzed by using Eosine Methylene Blue Agar method (EMB) in April and May field visits and HiChrome Coliform Agar method in December and March. Moreover, isotopes deuterium (2H) and oxygen-18 (18O) were measured using Triple- Liquid Water Isotope Analyzer (T-LWIA). All groundwater samples show high Total Dissolved Solids (TDS) indicating interaction with the hosting rocks (limestones, dolomite, and gypsum). However, surface water samples have low TDS which may suggest mixing with drinking network water (DNW). Groundwater shows high concentrations of Mg2+ and Ca2+ ions due to the interaction of water with limestones and dolomites followed by high SO42- and Cl- concentrations driven from gypsum and halite (present in Aruma Formation), respectively. Saturation index estimated for Al-Ansab water indicates that groundwater precipitates calcite and dolomite due to oversaturation with Ca2+ and Mg2+ ions and can further dissolve. Biological tests of Al-Ansab water concluded that there is no sewage leak to Al-Ansab groundwater associated with septic tanks, but rare contamination presents on the surface from animals and surficial activities. Isotope analyses showed that causes of groundwater level rise are both anthropogenic and natural. The leak from DNW has affected localized areas, whereas rainfall along with impermeable subsurface geology has affected groundwater level rise in a regional scale. The presence of the impermeable units in the Aruma Formation and the mar units in the tertiary sediments has significantly impaired the downward gravity flow and retained water within the thinner overlying alluvium.