دانلود رایگان مقاله لاتین مدل سازی ژئوشیمیایی از سایت الزویر
عنوان فارسی مقاله:
مدل سازی جریان و ژئوشیمیایی به عنوان ابزاری در نظارت شارژ مجدد مدیریت شده آبخوان
عنوان انگلیسی مقاله:
Geochemical and flow modelling as tools in monitoring managed aquifer recharge
سال انتشار : 2016
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مقدمه انگلیسی مقاله:
1. Introduction
Access to clean water is a growing global concern (Gale, 2005). In many countries, a growing population leads to both a higher need for food production and an increased demand for drinking water. These two pressures produce an increased demand for pure water and an increased use of groundwater reserves (Falkenmart and Widstrand, 1992; Shah et al., 2000). Replenishing groundwater reserves with surface water is a viable way of maintaining groundwater resources (Freeze and Cherry, 1979; Asano, 1985; Ma and Spalding, 1997; Gale, 2005). Managed aquifer recharge (MAR) is a general term for any engineering process where surface water is introduced into groundwater (Dillon, 2005; Page et al., 2012; Dillon et al., 2009). The main benefits of MAR are natural subsurface treatment for impurities, storage of water supplies (Grove and Wood, 1979), and groundwater replenishment, which may help reduce salinity (Dillon et al., 2002). Untreated surface water is generally not suitable for communal water supply use, often due to its high organic carbon content (Vartiainen et al., 1987; Lindroos et al., 2002; Grünheid et al., 2005; Kortelainen and Karhu, 2006). Organic matter can facilitate unwanted bacterial growth in water supply systems leading to odour and taste problems (Miettinen et al., 1999; Lindroos et al., 2002). Combining dissolved organic carbon (DOC) with chlorine disinfection can lead to the formation of mutagenic or carcinogenic compounds (Lindroos et al., 2002). MAR is widely used in Europe: for example, in Germany, Finland, Sweden and Hungary to remove DOC from surface water and make it more suitable for human consumption (Sundlof and € Kronqvist, 1992; Hatva, 1996; Van Breukelen et al., 1998; Kuehn and Mueller, 2000; Lindroos et al., 2002; Balderer et al., 2004; Grünheid at al. 2005; Kortelainen and Karhu, 2006; Kolehmainen et al., 2009). In North America, Australia and Asia the benefits of MAR for water quality have also been reported (Herczeg et al.,2004; Dillon et al., 2002; Vanderzalm et al., 2006; Moon et al., 2012; Graham et al., 2015). During artificial recharge, surface water is infiltrated into the topsoil of an aquifer by direct infiltration through infiltration ponds (Kortelainen and Karhu, 2006; Kolehmainen et al., 2009) or wells (Vanderzalm et al., 2006; Pavelic et al., 2007), by bank filtration (Grünheid et al., 2005) or by sprinkling (Lindroos et al., 2002). Previous studies have shown that the DOC content of the infiltrated surface water decreases most significantly early on along the flowpath (Frycklund, 1995, 1998; Greskowiak et al., 2005; Kortelainen and Karhu, 2006; Vanderzalm et al., 2006). During MAR, DOC concentration declines as a function of distance from the infiltration site (Kortelainen and Karhu, 2006; Kolehmainen et al., 2009; Grünheid et al., 2005; Lindroos et al., 2002). This is due to two possible processes: oxidative decomposition due to microbial activity in the aquifer, and sorption on the sediment without decomposing activity (Kortelainen and Karhu, 2006; Kolehmainen et al., 2009). Knowing the amount of oxidative decomposition of DOC, as opposed to DOC removal by sorption to the sediment, is important for understanding the purification process and the effects of MAR to the environment. In previous studies, 40e50% of the total DOC decrease has been attributed to oxidative decomposition (Frycklund, 1995, 1998; Kortelainen and Karhu, 2006; Kolehmainen et al., 2009). The oxidative decomposition of DOC can be monitored by examining the isotopic composition of dissolved inorganic carbon (DIC) in the MAR water, since the naturally occurring DIC and the DIC produced by the decay of organic material have a different isotopic signal. Decaying organic material adds a significantly lighter component into the DIC pool (Le Gal La Salle et al., 2005; Kortelainen and Karhu, 2006; Kolehmainen et al., 2010). This can be achieved by simple mass balance calculations when there are no external sources of carbon contributing to the DIC pool. Calcite-rich sediment and limestone are preferable environments for MAR for two reasons. Calcite dissolution is a beneficial process in MAR applications as the higher pH of water prevents corrosion of the water distribution pipes (Kortelainen and Karhu, 2009). Moreover, it has been shown by Pavelic et al. (2007) that calcite dissolution prevents aquifer clogging in MAR systems. Clogging leads to loss in permeability which shortens the lifespan of a MAR site (Vanderzalm et al., 2006; Pavelic et al., 2007). Treating the infiltrated water to prevent aquifer clogging is possible, but costly, and therefore the natural prevention provided by calcite dissolution is a definite asset (Pavelic et al., 2007). Calcite dissolution complicates the quantification of DOC decomposition as it introduces a third end-member to the carbon budget. The DIC in the infiltrating water is typically derived from the soil or the atmosphere and depleted in 13C compared to dissolved sedimentary calcite and also enriched in 13C compared to DOC (Veizer and Hoefs, 1976; Deines, 1980; Vogel, 1993; Schiff et al., 1997; Kortelainen and Karhu, 2009). Therefore, there are two opposing processes contributing to the change in the isotopic composition of DIC. Isotopic evidence can be used in geochemical modelling to constrain these opposing processes and their effect on the composition of DIC. In modelling the MAR system, the contribution of carbon from different sources can be identified if the isotopic compositions of the different sources are known. The model uses the isotopic compositions together with the chemical composition of the source and resulting waters to produce mixing ratios between the two types of carbon. A MAR site infiltrating surface water from the Kokemaenjoki € River located north of the city of Turku was commissioned in 2010 at Virttaankangas. Prior to the startup of the site, thorough studies were conducted on the sedimentological background and water chemistry (Kortelainen et al., 2007; Kortelainen and Karhu, 2009) and a three-dimensional model of the sedimentological and hydrogeological units containing the aquifer was developed (Artimo et al., 2003). The MAR site meets the main water supply needs of the Turku region with approximately 285 000 inhabitants (Artimo et al., 2003). The Virttaankangas site was selected to host the MAR site because of the vast groundwater reservoir in the aquifer, its proximity to the Turku area and the naturally high pH of the water, which is due to calcite dissolution (Kortelainen and Karhu, 2009). The main goal of this study was to quantify the oxidative decomposition of DOC and the dissolution of calcite to estimate the effectiveness of the MAR site at Virttaankangas. As a part of this, we had to determine the point in time when DOC decomposition commenced. To achieve these goals, we used a groundwater flow model to determine the flow routes from infiltration areas to the production wells and then the hydrogeochemical modelling program PhreeqC (Parkhurst and Appelo, 2013) to determine the onset and the amount of DOC decomposition in the aquifer.
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