Most people in Bengkalis island use groundwater as their primary resource, but brackish water discovered in the wells located near the shore as well as in the distance. A hydrogeology study through geochemistry and isotope aspect conducted for the first time to identify the effect of saltwater intrusion (SWI) on groundwater. Na > Mg > K > Ca were the most dominant cation sequence in groundwater, while Cl > HCO3 > SO4 was the most dominant for anion. NaCl is the main type of groundwater in shallow aquifer and NaCl & NaHCO3 both became the most dominant type of groundwater in the deeper aquifer system. Most of the groundwater samples from dug wells indicating the weak acid exceeding strong acid. Meanwhile, the groundwater samples from bore wells dominated by alkalis exceeding alkaline earth. SWI analysis shows 36.4% of groundwater samples from dug wells and 40% of bore water samples experiencing the salinization process. From isotope analysis, the groundwater samples found to be originated from the higher elevation or distance source. Meanwhile, there are lighter 2H values in the samples that are indicating the groundwater flown in the deeper part of the aquifer.

The level of slope influenced by the condition of the rocks beneath the surface. On high level of slopes, amount of surface runoff and water transport energy is also enlarged. This caused by greater gravity, in line with the surface tilt from the horizontal plane. In other words, topsoil eroded more and more. When the slope becomes twice as steep, then the amount of erosion per unit area be 2.0 - 2.5 times more. Kuok and surrounding area is the road access between the West Sumatra and Riau which plays an important role economies of both provinces. The purpose of this study is to map the locations that have fairly steep slopes and potential mode of landslides. Based on SRTM data obtained, the roads in Kuok area has a minimum elevation of + 33 m and a maximum + 217.329 m. Rugged road conditions with slope ranging from 24.08 ° to 44.68 ° causing this area having frequent landslides. The result of slope stability analysis in a slope near the Water Power Plant Koto Panjang, indicated that mode of active failure is toppling failure or rock fall and the potential zone of failure is in the center part of the slope.

Vertical Electrical Sounding (VES) method is one of many methods in geophysics that was conducted in this research at Toro Jaya, Langgam, Riau. The aim of this research is to investigate and locate the depth of groundwater layer (aquifer). Data acquisition of VES was carried out using GEOCIST with Schlumburger configuration of electrode. There are four VES point in this research with various length of cable ranging from 135 m up to 200 m. From four VES point shown two different of resistivity value range. Very low resistivity value with range 0,34 – 0,9 Ωm interpreted as aquifer layer. The aquifer layer of research area divided into two types which are unconfined aquifer and confined aquifer. Meanwhile, another resistivity value ranging from 1,69 – 7 Ωm was interpreted as clay layers.

Research area is around Tanjung Balik, Koto Baru Sub Base, Lima Puluh Kota District, West Sumatra Province. Located along the highway Km 10-15 Riau – West Sumatra and the coordinate around 00˚08'40 '' LU - 0˚11'20 '' N and 100˚45'20 '' BT - 100˚47'00 '' BT. The purpose of research to identify petrography, microstructure, types of landslides and the geological condition. The methods using polarization microscope, stereography, landslide identification survey and geological mapping. The result of study shows the petrography analysis of lithology of study area are classified into three types of rocks are Feldspathic Greywacke, Lithic Arenite, and Slate. Microstructures trending system show the foliation structure that is relatively Southeast-Northwest. Types of landslide which dominates in the research area are debris avalanche and translational landslide. Geological analysis show some of rock units are classified into two units: Sandstone Unit and Slate Unit. Sandstone Unit spread in the northern part of the study area, while Slate Unit spread in the southern part of the study area. The characteristics of these rocks showed Pematang Formation.

Saltwater intrusion becomes a common problem in coastal area. Northern coast of Bengkalis Island in Riau, Indonesia that contiguously to Malacca Strait is experiencing the problem particularly in Selat Baru area which considered as developing area and most of the people live close to the sea. Dug well is the main source of groundwater that had been used by the population in Selat Baru and as the increasing of land occupation, the demand of clean water is rising up followed by the number of dug well. A study of groundwater monitoring was conducted to identify the zone of saltwater intrusion. Field observation had done by measuring the water level and physical parameters of groundwater such as taste, pH, electrical conductivity (EC) and total dissolved solid (TDS) from 110 existing dug wells. Some conus feature had found from groundwater elevation map which indicated lower water level caused by excessive groundwater pumping. Generally, pH shows values from 6-8 that was still in range of water quality standard, but there are 20 wells (18%) that have pH below the water quality standard (slightly acidic water). Similar pattern had been observed from Electrical Conductivity (EC) and Total Dissolved Solid (TDS) map, higher value of EC and TDS was dominant in the northern part of study area and lower value in the south. Groundwater taste map also revealed the identical condition with EC and TDS map which dominated by brackish and saline water in the northern part. Therefore, the study area had been divided into two zones of groundwater saline water zone possibly caused by the saltwater intrusion in the northern part and freshwater zone from the center to the southern part.