Water quality assessment and recommended monitoring for ASR pilot location Nga Bay

The project ASR application for Domestic Fresh Water Supply in the Vietnamese Mekong Delta was made possible by a grant under RVO's Partners for Water program. In the Partners for Water project "ASR application for Domestic Fresh Water Supply in the Vietnamese Mekong Delta," the ambition is to build an Aquifer Storage and Recovery (ASR) pilot location in Hau Giang. This report provides an assessment of the water quality from the drinking water treatment plant in Nga Bay and the water quality of the native groundwater at the start of the pilot, to address the possible risks connected to ASR. The report takes the more generic overview of the monitoring plan (Report No 1) as the basis and specifies this to the pilot location by analyzing water quality data and giving site-specific monitoring recommendations. Assuming treated water (before chlorination) will be used for the ASR pilot, all measured water quality parameters are far below Vietnamese standards for drinking water and groundwater. When standards for MAR projects from other countries in the world are considered, no water quality problems are expected. Prevention of clogging risks is an important goal of the ASR pilot. The relevant water quality parameters (such as TSS, iron, and TOC) indicate that clogging risks are low. The water treatment prior to infiltration also decreases the risk of clogging. Over-infiltration can further decrease potential risks of clogging. Pollution of native groundwater in target aquifer qp1 is not likely, because of the great depth and presence of protecting clay layers, and the fact that pesticides were not found in the existing back-up wells. Still, we recommend monitoring pesticides. Based on scarce scientific literature, their presence in the treated surface water is likely. In addition, pesticides form a very broad group of compounds, and not all of them will be removed by the drinking water treatment. Next to pesticides, compounds of emerging concern can also pose a risk of contamination of the aquifer, but no data is available. From this point of view, it is important to also keep control of the infiltrated and recovered volumes, to avoid spreading of infiltrated water. Risks of deteriorating water quality during storage cannot be excluded. The pilot itself will provide more insight into these risks. Risks exist on mobilization of metals like Mn and As although concentrations in the native groundwater seem rather low. In addition, risks of microbiological growth must be assessed by careful monitoring during the pilot. By using non-disinfected treated water, the risk of forming undesired chlorinated compounds in the aquifer can be minimized. A point of attention is the salinization risk from aquifers above or below the target aquifer. NAWAPI data from Q607 do not indicate salinity issues in the adjacent aquifers, however, DONRE data do show the presence of saline water bodies in the shallow aquifers. Although it is expected that risks of contamination or clogging are limited, it is still important to monitor the infiltration pilot well, as the surface water quality is expected to vary over time due to, e.g., seasonal effects. In addition, hydrogeochemical reactions in the groundwater system might change over time and are not exactly known beforehand. Finally, water quality data on input and produced water can give information on the processes during infiltration. Therefore, a detailed monitoring scheme is recommended, including a list of parameters to analyze, sampling points, and sampling times.