Bioassessment of Dry Season Water Quality in the Ping River around Chiang Mai City, Thailand

A longitudinal transect along the Ping River around Chiang Mai city was made during the dry season of 2010 to assess the extent and location of human impacts on water quality. The grab samples showed water quality within the urban area was poorer than upstream. However, physico chemical analyses did not differentiate levels of human disturbance in weir affected by the urban development. Bioassessment using benthic diatoms and littoral macroinvertebrates did differentiate sites within the city and showed the lower Chonlakhan Pinij weir pool was the most disturbed site. Regional biotic indices for macroinvertebrates ((BMWPTHAI) and diatoms (Mekong Disturbance Index) were more reliable and discriminating than local (Ping River) the indices. Overall, the grab sample data showed dry season water quality in the upper (Tha Wang Tan) weir pool within the Chiang Mai city has improved over the past twenty years. But the health of the Ping River downstream of the Mae Kha discharge likely deteriorated over the same period. Future water pollution control efforts around Chiang Mai should consider the impact of the Mae Kha canal on the Ping river.