Investigation on the Quality of Groundwater Resources of South of Birjand Plain Using Mass Balance Techniques

Groundwater is of important drinking water resources. The quality of those water resources which are not affected by human activities, is characterized by local geological conditions. The present research was conducted in order to investigate the quality of water resources located in the south of Birjand plain, South Khorasan Province, using mass balance techniques. This research dealt with the quality assessment and the distribution pattern of physico-chemical parameters (anions and cations, pH, EC, TSS, TDS, and TH) in the groundwater resources in the region resulting in indentifying the contamination resources. Also, the origins of anions and cations in the groundwater resources as well as their connection with the geological characteristics of the region were evaluated by employing the mass balance techniques. The water sampling was performed systematically based on the distribution wells over the plain, with regard to the distances, and perpendicular to the topographical lines from the heights toward the lower plains down to the main river, 3 kilometers apart across the plain. In average, one third of whole wells in the study area were selected for sampling. The samples were taken during the end of dry season (October 2010) from 27 wells, 2 Qanats, and 1 spring. The cations (Na +, Ca+2, Mg+2, and K+), anions (HCO- 3, SO 4 2-, CO 3 2-, and Cl-), TSS, TDS, pH, and the total hardness of the water samples were measured in the Water, Soil, and Plant Laboratory, Kavosh Ab Shargh Khorasan Jonobi Engineering Services Co. To assess the quality of the water resources in the region, in this research the physicchemical properties of water samples were compared with ISIRI 1053 to identify the contaminated wells. The hardness of the samples was measured and the water resources types were identified using Piper Diagram. Also, through calculating the correlation factors and mass balance, hydrogeochemistry of water resources and probable origin of cations and anions were assessed. The results showed that 50%, 30%, and 10% of the groundwater resources are contaminated with magnesium, sodium, and chloride respectively. The petrological studies verified the presence of ophiolite sequence. A set of ultramafic rocks and basic magma (spilites) with wide developments in southern heights of Birjand plain which had the most effects on the water quality of the region. The water resources contaminations in the region may be due to the presence of listonites from the alteration of ultramafic rocks existed in highlands, upstream of the groundwater resources. 96% of groundwater resources are classified as very hard water. The types of groundwater vary from carbonated calcic in highlands to chloride sodic in downstream and east of the region due to high solubility of Na + and Cl- existing in the soil. %36.6 of water resources were chloric-sodic and the rest were classified as chloric-magnesic, bicarbonatesodic, and bicarbonate-magnesic. The correlation analysis of the parameters indicated that sodium has a very high correlation with chloride, potassium, and sulfate. The positive correlation of Mg 2+ with Ca2+ (+0.614) is an indication of the same origin for both ions. Also, EC with TH (+0.710) and Ca 2+ (+0.710) and TH with TSS (+0.641) show positive correlation. In general, these positive correlations indicate the common geogenic origin of cations and anions in water resources of the region. WATEVAL software was employed for calculating the mass balance using the concentration of anions and cations to find the effective parameters and origin of ions in the water resources. These calculations verify and emphasize on the influence of geological conditions of the region on hydrogeochemical properties of water resources of the plain. Mass balance analysis denotes that the origin of sodium and potassium could be the ion exchange process originated from weathering of mafic rocks (spilites) while the calcium may be released due to weathering of plagioclases of sub-volcanic mass, ferromagnesian minerals, carbonated listonite, and dissolution of limestone in the region.