دانلود رایگان مقاله نظارت شارژ مجدد مدیریت شده آبخوان و مدل سازی جریان و ژئوشیمیایی

abstract

Due to a growing world population and the effects of anthropogenic climate change, access to clean water is a growing global concern. Managed aquifer recharge (MAR) is a method that can help society's response to this increasing demand for pure water. In MAR, the groundwater resources are replenished and the quality of the recharged surface water is improved through effects such as the removal of organic matter. This removal occurs through mechanisms such as microbial decomposition, which can be monitored by studying the isotopic composition of dissolved inorganic carbon (DIC). Nevertheless, the monitoring can be difficult when there are other factors, like dissolving calcite, affecting the isotopic composition of DIC. The aims of this study were to establish a method for monitoring the decomposition of organic matter (dissolved organic carbon e DOC) in cases where calcite dissolution adds another component to the DIC pool, and to use this method to monitor the beginning and amount of DOC decomposition on a MAR site at Virttaankangas, southwestern Finland. To achieve this, we calculated the mean residence times of infiltrated water in the aquifer and the fractions of this water reaching observation wells. We conducted geochemical modelling, using PHREEQC, to estimate the amount of DOC decomposition and the mineral reactions affecting the quality of the water.

6. Conclusions

The main aim of this study was to quantify the oxidative decomposition of DOC and to determine when the oxidative decomposition of DOC started in the aquifer. For this purpose the mean residence time (MRT) and the simulated shortest travel times (STT) of the infiltrated water were determined. At the Virttaankangas MAR site, the MRT of the infiltrated water was from 15 to 40 weeks. Decomposition of DOC commenced with a higher infiltration volume. The total reduction of the concentration of DOC was 30e50% of the total DOC contents. Of this, between 30 and 80% was due to oxidative microbially induced decomposition. These results show that the Virttaankangas MAR site functions well, and that the amount of DOC decomposition correlates with the MRT. The high pH environment at the Virttaankangas site seem to delay the beginning of DOC decomposition. It is concluded that seasonality in the d18O content of the infiltrated water can be used to determine the MRT in the system. The mean residence time can differ significantly from the STT of the infiltrated water and these do not always correlate: a short STT does not necessarily mean a shorter MRT. In a MAR system where oxidative decomposition of DOC is coupled with calcite dissolution, carbon isotope mass balance combined with geochemical modelling may be applied to estimate the quantities of oxidative decomposition of DOC. The reasons for the delay in the oxidative decomposition of DOC in a high pH environment require further study