Hydrochemical and Isotopic Characteristics of Groundwater in the Lampang Basin, Northern Thailand

Abstract

Analyses of hydrochemical and stable hydrogen and oxygen isotopes, 2H and 18O, were conducted on 76 water samples collected during September and October, 1997, and during January and December, 2002. The set included 10 rainwater samples, 16 river water samples, 2 surface water samples, and 48 groundwater samples, all within the Lampang basin. The analyses were done to document the chemical and isotopic characters of the natural waters, and to determine the origin of groundwater in three different aquifer units. These aquifer units are Holocene unconsolidated sediments, Pleistocene unconsolidated sediments, and Tertiary consolidated and semi-consolidated sediments. Hydrochemical data are classified on the basis of dominant ions by mean of Piper-trilinear diagram as hydrochemical facies. All the groundwater samples from the three aquifer units are similar in hydrochemical facies. This indicates that all of the groundwater is recharged by chemically similar waters, i.e., the groundwater is infiltrated by the same source of recharge water. It was not possible to define a meteoric water baseline for the Lampang basin. So, the meteoric water baseline for Thailand, δD = 7.35 δ18O + 6.11, R2=0.972, was adopted as the Lampang basin meteoric water baseline. Isotopic analyses of groundwater from the three aquifer units are not significantly different in their δD-δ18O and are very similar to local precipitation. This suggests that all of the groundwater in the system is recharged by isotopically similar water from local precipitation and from infiltration water that is not fractionated during passage through the aquifers. Equally, there is little or no effect from interactions between the water and rocks. The isotopic composition of the river water samples is enriched in heavy isotopes relative to the groundwater samples. The possible explanation for this is, that the river water may be derived from groundwaters that have been mixed with water derived from dams.