Water for people.pdf - WHO Thailand Digital Repository

3 9 4 / P I L O T C A S E S T U D I E S : A F O C U S O N R E A L - W O R L D E X A M P L E SChao Phraya River Basin, ThailandTable 16.5: Groundwater storage and renewable water resource of thesub-basinsGroundwater Renewable waterstorageresourcesGroundwater basin (million m 3 ) (million m 3 )Chiangmai-Lampoon 485 97Lampang 295 59Chiangrai-Payao 212 42Prae 160 32Nan 200 40Upper Chao Phraya 6,400 1,280Lower Chao Phraya 6,470 1,294Total 14,222 2,844The calculation is based on the assumption that the amount of groundwater stored dependson the change of water level, the area of the aquifer and the storage characteristics, whichvary with the geology of each area – unconfined, confined or semi-confined. The Upper andLower Chao Phraya groundwater basins are by far the largest ones: they store about 90percent of the overall groundwater resources of the case study area. The table assumes thatonly 5 percent is renewable each year, a very small amount of the total in the basins.artesian conditions exist. The continued decline in levelsindicates that abstraction is not in balance with recharge. Inunconfined aquifers, abstraction of resources in excess of naturalrecharge normally leads to a much lower rate of water leveldecline than that of confined aquifers.Groundwater qualityThe main chemical constituents affecting groundwater quality aresodium and chloride. The average salinity of the groundwater inthe unconfined aquifers shows a general increase in thedownstream direction, with the exception of the Ping catchmentwhose lowest salinity level is comparatively high for its uppercatchment location. The groundwater with the lowest salinitycomes from the Wang catchment. Nitrate concentrations arealmost invariably low in all catchments. The extent to whichchemical quality is affected by contamination is not known,except in some specific areas.Rainfall variationThe basin climate consists of long, distinct dry and rainy seasons.These give rise to typical water problems: water shortages, floodsand water pollution. These problems can become the source ofenvironmental and socio-economic problems: for example, watershortage in Bangkok leads to overpumping of groundwater andsubsequent land subsidence and flooding. In the basin’s ruralareas, encroachment on forest land leads to excessive erosion,extreme floods and landslides. Water pollution is widespread inurbanized sections of the Chao Phraya River and its tributaries.Recently, due to encroachment on forested areas, flash floodsand landslides have also become frequent.Water qualitySurface water qualityA study in 1997 (Binnie & Partners, 1997) reviewed the waterquality data for the Central River basin, which is routinelymonitored by the National Environmental Board (NEB), thePollution Control Department (PCD) and the Ministry of PublicHealth (MOPH). Results indicated that of the major rivers in theLower basin there was evidence of heavy pollution in both theChao Phraya and Tha Chin Rivers, while overall water quality wasacceptable in the Pasak and Sakae Krang Rivers. The ChaoPhraya River exhibited serious organic and bacterial pollutionthat was a threat to many species of aquatic life. Similarly, waterquality in the Tha Chin River was heavily degraded, due to thecombined discharges of industrial, domestic and rural inflow.FloodingFloods are a natural phenomenon in the Chao Phraya River basinand while residents have adapted their lifestyle to deal withannual flood occurrences, they cause significant economic losses.The major causes have been the decline of flood retention areasand the confinement of flood plains due to increasingdevelopment, the rapid urbanization around Bangkok, the growthof provincial cities and the intensification of agriculture.In recent years, the government has had some success inreducing flooding through the construction of multi-purposereservoirs, dikes, and other flood control infrastructures. Thecontainment has, however, resulted in a higher overall flood riskas elevation levels are reached more quickly.Human impacts on water resourcesIn general, the basin is entering a critical period where smallchanges in the hydrologic conditions can create large socioeconomicdisturbances. Due to the increase in population, it isunavoidable that new settlements will arise in areas where watermanagement is difficult. The human impact on water resources,and vice versa, is visible throughout the basin. Native plants actingas surface cover on native land are being destroyed at an alarmingrate, causing flash floods, erosion and landslides. The construction