Seepage Chemistry Investigations
When dams are sited on karst or foundations containing water soluble minerals such as gypsum, anhydrite, calcite, or dolomite, reservoir water and seepage can be chemically analyzed and compared with seepage flows to determine how fast soluble rock is dissolving. Seepage chemistry data can also be used to determine if increases in seepage concentrations are due to mixing with another local groundwater, or other processes unrelated to mineral dissolution. CEO Doug Craft has experience from seepage chemistry studies conducted at several major dams including Glen Canyon Dam (Arizona), Horsetooth Dam (Colorado), Clark Canyon Dam (Montana), and Deer Flat Embankments (Idaho). While seepage chemistry should never be used as the only way to assess seepage problems, it can provide cost effective and important information - such as void formation rates - unavailable from other monitoring methods.
In a typical seepage chemistry study, water samples are collected from the reservoir, piezometer wells, toe drains, and surface seeps at low and high reservoir surface elevations - usually over a period of several years. Field variables temperature, conductivity, and pH are measured in samples when collected. The samples are then analyzed by a lab for major ions, organic carbon, and several trace elements. The basic evaluation of seepage chemistry depends on evaluating the differences between reservoir concentration and seepage concentrations. The accumulated chemistry data may then be interpreted using geochemical data analysis methods and comparison of the chemistry to piezometer elevation data, measured seepage flows, the local groundwater hydrology at the dam. and the geology and mineralogy of the foundation and abutments.
Sample seeps where flow is measured, such as the calibrated weir at Clark Canyon Dam seen to the right. Below to the left is a groundwater sampling pump used to collect water from a piezometer well at Horsetooth Dam.
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It is critically important to follow proper sampling procedures, making sure the well is adequately purged before sampling and that conductivity, pH, and temperature are measured during the sampling process. Improperly collected samples often lose any meaningful interpretative value and waste the cost of chemical analyses and field crew time at the site.
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If good samples are collected, the data can interpreted using a variery of geochemical visualization and analysis methods. The Stiff diagrams on the right compare reservoir concentrations (inner green polygons) and seepage concentrations (outer blue polygons) on the same plot. This data visualization enables quick and intuitive identification of water types related to geology along different flow paths, and changes that have occurred over time. |
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To the left are Stiff diagrams annotated on an engineering plan map of Horsetooth Dam that show different chemistry (polygon shapes) for each piezometer located in the embankment and alluvium. The weir sampled downstream of the seepage pond (right) suggests a mixing of different seepage waters. Similar plots annotated on geology cross sections can be used with plan maps to help establish flow paths for seepage. |
Other reasons for changes in seepage concentrations must be evaluated before mineral dissolution is assessed. Possible mixing of local groundaters and hydrologic effects must be considered. Application of a groundwater flow model at Glen Canyon Dam suggested that increased concentration seepage emerging downstream of the dam represented higher concentration reservoir water seeping horizontally at depth around the abutment over the period of several months. The similarity to same elevation reservoir concentrations suggested that mineral dissolution was not a likely process. |
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The last step before estimating the void formation from mineral dissolution is to apply a geochemical analysis method called a mass balance model. This approach develops a set of balanced chemical reactions consistent with the field data, seepage chemistry, and the geology and mineralogy at the site. Other common processes that can increase seepage concentrations include bacterial oxidation of organic food carbon during seepage transit and ion exchange on clays. Once these factors are calculated in the mass balance model, the remaining increases may be attributed to mineral dissolution.
The methods Doug adapted and applied on seepage chemistry investigations are summarized in detail in the Seepage Chemistry Manual, available for free download as a PDF file. Other technical reports that apply these methods are also available for from the reports page on this site.