500/6 Bizpoint 10
Canal Rd, Maehia, Chiang Mai 50100
Ph : 093-698-4884
Email : reception@chiangmaidoctor.com
Groundwater in the
Chiang Mai - Lamphun Basin
This webpage is a short description of aquifers in the Chiang Mai basin. Further information and references can be found in 'Floods and Water Management in Chiang Mai and the Upper Ping Catchment (Pirard, 2025)'.
Go directly to:
Previous Page: Historical floods in Chiang Mai
Next Page: Infrastructure in the Ping river catchment
Geological context
The Chiang Mai – Lamphun basin is a 70 x45 km2 plain surrounded by mountains. It is filled by more than 1200 meters of Tertiary and recent river sediments deposits on old Paleozoic rocks that form the basement and the mountain ranges surrounding Chiang Mai (see geology). Various aquifers are held into these sediments and while old hydrogeological models considered aquifers to be limited by the distribution of paleoterraces, the reality is a bit more complicated than that with several distinct sedimentation domains producing various groundwater sources down to 200 meters.
The general situation is that aquifer recharge occurs mainly on the edge of the basin in terrace deposits from July to October and slowly flows towards the central part of the basin to eventually discharge into rivers through floodplain sediments. The age of the water in shallow aquifers accessible for exploitation is relatively young, between 5 and 40 years old.
Figure 1. Hydrogeological map of the Chiang Mai - Lamphun Basin. Yellow terrains are Tertiary and recent sedimentary rocks deposited by the Ping river. Other colours (pinks, green, grey, red & orange) are other Paleozoic hosted aquifers. On the right are three cross-sections through the Chiang Mai - Lamphun basin (modified from Taweelarp et al., 2021).
Aquifers
The central fluvial channel is made of Quaternary river sediment of unconsolidated sand and gravels with minor silty strata. There is no lateral extension that allow correlation between boreholes but the shallow aquifer is between 0 and 50 m deep sometimes extending to 70 m. The hydraulic conductivity is high, allowing significant output out of these wells. However, most of the aquifer is unconfined which means that pollution from the surface can easily reach the water table and the aquifer is therefore labelled as vulnerable. In the southern part of the basin (e.g. San Pa Tong), a few meters of clay makes it partially confined. A deeper aquifer is also present in the basin but the piezometric head is significantly lower than the shallow aquifer.
The Mae Kuang alluvial fan is a domain under San Sai, Doi Saket and San Kamphaeng made of layers of various coarse and fine sediments deposited by the Mae Kuang and Huay Bon rivers. The average aquifer exploitation depth is 50 m with variable output. The water is of good quality although the southern portion has higher level of turbidity.
The Mae Wang – Mae Khan sub-basin is another alluvial aquifer created by continuous subsidence in the area. Sediments are sand and gravels with variable water output.
Figure 2. Subdivisions of the Chiang Mai – Lamphun Basin aquifers overlaid on a topographic map. Blue is surface water and dotted purple line is the limit of the basin aquifer
Colluvial aquifers are present east of Mae Kuang and the southern margin of the basin, Huay Bon and the North-West of Chiang Mai. It is formed by slope wash accumulating sand and gravels at the base of hills surrounding the basin. The aquifer itself can be thin and it is not uncommon for wells to reach the bedrock at a couple of hundred meters deep. With its low capacity and conductivity, the colluvial aquifer seems to be over-exploited and lowers around 1 m per year. It also has high fluoride concentration.
Low terraces are sediments deposited in the last million year that can reach several hundred meters in thickness. Around the floodplain, the low terrace aquifer is around 30 m and unconfined while there are deeper confined aquifers under the central alluvial channel and form an aquifer than can reach 150 meters of thickness. Sediments are clay layers with some sand and gravels, providing moderate to high yield with a water of good quality.
High terraces were formed in the past few million years and are mostly present on the western margin. It is several hundred meters of poorly sorted sand and gravel beds alternating with silt and clay. Occasionally, the aquifer is unconfined and can also be significantly kaolinised, reducing well yields. Variation in piezometric head during exploitation indicates a low recharging rate but the water is fair to good quality.
Figure 3. Groundwater quality map for total dissolved solids and fluorine for water pumped between 10 and 80 m below ground
Groundwater composition
The groundwater in Chiang Mai is generally of good quality. It has a low salinity but is relatively hard due to abundant limestone in the upper part of the basin. Further south, the water shift from a Ca-HCO3 equilibrium to Na-HCO3 due to cation exchange with clays. Clays also can provide higher fluoride concentrations and there are some part in the south-east of the basin and around Lamphun and in deeper aquifers where fluorine exceed health recommendations.
Iron and manganese have levels above 1 mg/l and 0.5 mg/l respectively in two thirds of the wells in the basin and while not being a health issue, it can be a technical problem as it might clog pipes and filters after some time. The central alluvial channel, where most wells are, have low fluorides and iron levels as the aquifer is quite oxygenated, limiting dissolution. Dissolved solutes (TDS) are overall generally acceptable but increase towards the eastern edge of the basin.
The rapid urbanization of Chiang Mai and industrialization of Lamphun has considerably increased the risk of over-exploitation but also contamination. While aquifers with low permeability soils (thick layers of clay and silt) as well as perched aquifers and artesian water tables are relatively protected, in many places, including most of the central alluvial channel, aquifers are unconfined and the risk of surface pollution is significant, especially when industrial plants and landfills with hazardous management are not particularly monitored.
Figure 4. Groundwater extraction potential in the late 90s with a distribution of over 2500 registered wells (as of 2019).
Exploitation
Most wells are between 10 and 80 m deep and the highest exploitation is in the central alluvial channel and some localised area in the Mae Kuang alluvial and Mae Wang – Mae Khan sub-basin. Recharge rates vary between 17-25 mm/yr in colluvial aquifers to 200-300 mm/yr in the central aquifer.
Following droughts in the 80s and with the explosive urbanization of Chiang Mai, the number of wells has increased significantly in the past few decades. In the late 90s, 21800 drilled well reports existed for Chiang Mai, mostly for private and/or agricultural purposes, with only 1117 government wells that were properly monitored. In 2019, this number is 2568 wells and a number at least 10 times higher of unregistered exploitation wells. This leaves a lot of uncertainty of how much groundwater is extracted for aquifers in Chiang Mai.
Based on piezometric levels in monitored wells, through the seasonal variation of water level which can be up to 10 meters or more, there is a medium term decrease in levels due to overexploitation. While the aquifer near Chiang Mai city centre appear to allow further exploitation, some other areas in Hang Dong, San Patong, San Kamphaeng and Mae Rim show significant level drops in the past two decades.
Figure 5. Isopiestic map of the CML basin and piezometric head records in 3 different locations between 2007 and 2020 showing an apparent steady lowering of the water level (modified from Taweelarp et al., 2021).