Chromium has two common forms based on oxidation state namely Cr(III) and Cr(VI). Cr(III) are more insoluble (<20μg/L) between pH 7 and pH 10, with a minimum pH solubility of about 1μg/L at pH 8.1 pH values in Cr(VI) tends to be acid in pH 6–7. IARC classifies Cr(VI) as a carcinogenic and harmful substance to humans. The high concentration of chromium in drinking water in Spain can be inferred from piping and fittings tool.2 In Indonesia, a high accumulation of Cr(VI) can be found around the river near the interaction of Talawaan–Tatelu, North Sulawesi.3 Residents exposed to Cr(VI) are confirmed to have digestion, dermatological and hematological abnormalities.4

Silicon dioxide is a dominant minerals and easily discovered in mountains area. This compound may undergo different dissolution or erosion in the environment. Silicon dioxide is a toxic compound and frequently accumulated around the cement plant.5 Inhalation and oral pathways are the primary pathways of human exposure to silicon dioxide. Potential effects of long-term exposure to c-silica include silicosis, COPD and kidney disease. Soluble silicon dioxide in drinking water possibly increase health problems which is correlated to acute gastrointestinal disease, where silicon dioxide was found at high concentrations in hemodialysis patients and kidney disorder.6

The risk level of drinking water is determined based on the HQ (Hazard Quotient) ratio which used to estimate ecological risks of a media.7 HQ values are assessed based on contaminant concentrations in research area and Minimum Risk Level (MRL), a dosage where it should represent long-term effect of contaminants exposure.8 This study aims to assess the ecological risks associated with human consumption of well water containing Cr(VI) and SiO2. The local should aware about long-term effects of drinking water consumption.

The maximum WHO contamination level for total chromium in drinking water is 50μg/L. The safe concentration for drinking water consumption of Cr total chromium in Canada is 0.1mg/L.10 Based on Duckham’s research, Cr(VI) minimum risk level is 0.07μg/L. This value based on increased risk to one per million of the average person affected by cancer as a result of drinking well water with Cr(VI) for approximately 70 years.16,17 NOAEL or Screening Benchmark values for Cr(VI) in this study were taken from the CPHGCDW which collected levels of 0.002mg/L.14 The level of health protection taken through toxicity experiment in female mice in the NTP study.18

Heavy metals can pollute wells through the movement of groundwater and surface water.15 Leaching processes from topsoil and rock soils are natural sources and play an important role in the flow of chromium to groundwater. The cement industry produces heavy metal dust, gas and particulate matter.4 Research in cement factories in Calabar, consist of the chromium and cadmium content of all water samples (surface water, wells, boreholes, and streams) which found that Cr(VI) concentrations far above WHO recommendations in the region.17 Generally, Cr(VI) concentration in drinking water is about 0.01 to 0.05mg/L and could be more higher in region with deposited of fly ash.19Table 2 shows some concentration data, with different source and area that polluted with Cr(VI) in groundwater.

Cr(VI) can be produced naturally in soil and sediment. Solid reactions with solubility and period time are enough to determine the concentration of Cr(VI) in groundwater.20 Cr(VI) and other heavy metals that found around the industrial area is strongly associated with anthropogenic contamination.21

There are various data of genotoxicity and mutagenicity of Cr(VI) in vivo and in vitro. In specific, chromium in its hexavalent form is considered to be a carcinogen.26 Based on Fig. 3, three stations with potential exposure of Cr(VI) through the consumption of well water containing Cr(VI) are station 10 (HQ=3.05), station 11 (HQ=1.45), and station 4 (HQ=1.1). Whereas in the other stations, the HQ value is still below 1 (low risk).

Figure 3.

HQ of Cr (VI) and SiO2.

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Silicon dioxide does not react directly with water because of its complex covalent structure. In high temperature and concentrated solution, silicon dioxide possibly reacts with sodium hydroxide.27 A feasible positive correlation was found for SiO2 which showed that more silicon dioxide was dissolved by high-temperature water. The risk of well water consumption around the cement industry and karst mines should be limited to reduce human health effects in the future. The quantity of silicon dioxide in well water varies from 1 to 30mg/L.28

Silicon dioxide is one of the compounds that commonly found on Earth's surface. Land expansion will reduce environmental quality due to the entry of silicon dioxide and other toxic minerals into groundwater which is connected to all water sources from karst mountains and resident wells. This process will poses a health hazard to the surrounding population. Data of MRL of silicon dioxide through oral from male rat experiments is 2.030mg/day.29 This is the minimum dose where gastrointestinal problems, hematology and lethality are present. From Fig. 3, it can be seen that SiO2 has higher HQ values than Cr(VI). All stations have HQ>1. This indicates that mostly areas have a moderate level of ecological risk. Only two stations with HQ value are more than 10, both of them located in Tukamasea Village.

The pH trends throughout the station are in neutral (slightly acid) conditions. Other mineral contents may affect the composition of hydrogen ions so the obtained pH is less acid due to the rock weathering process.30 In a region with the highest Cr(VI) content, pH 5.52 was recorded. In contrast, the station with the highest silicon dioxide concentration was slightly alkali (pH 7.74). The possible way to reducing Cr(VI) and SiO2 pollutants in environment are reductions, coagulation, and filtration process. Low base ion exchange and strong base ion exchange are effective technologies for removing Cr(VI) from drinking water. The risk of water consumption of wells around the cement industry and karst mines should be limited to reduce acute and carcinogenic effects in the future.

The HQ was calculated to assess the well water quality of 14 samples from Maros Karst area, Indonesia, and health risks caused by exposure to Cr(VI), and silicon dioxide were also considered. Understand and aware contaminants origin from groundwater through the leaching, geological and anthropogenic processes are necessary. The obtained HQ values shall be considered as an evidence base and provide valuable information to identify the source of chemical species. This research is really important in managing and preserving valuable groundwater resources.

The authors declare that they have no conflict of interest.