GROUNDWATER QUALITY   
 

    A.  DISSOLVED SUBSTANCES

        1.  TOTAL DISSOLVED SOLIDS 

              a.  TDS:  the total mass of dissolved substances, usually
                   expressed in milligrams of solid per liter of solution
                   (mg/L), which is equivalent to parts per million (ppm)

              b.  Potable (i.e., drinkable) groundwaters typically have
                   TDS values in the 100 to 500 mg/L range
  

        2.  MAJOR CONSTITUENTS

              a.  Substances present in concentrations above 5 mg/L  

              b.  Typically, the major constituents in groundwater are
                   Na⁺, Ca²⁺, Mg²⁺, HCO₃^-, Cl^-, SO₄^2-, and SiO₂^o  
 

         3.  MINOR CONSTITUENTS

              a.  Concentrations range between 0.01 mg/L and 5 mg/L  

              b.  Typically, the minor constituents in groundwater are
                   K⁺, Fe²⁺, Sr²⁺, F^-, B, and NO₃^-
 

        4.  TRACE CONSTITUENTS

              a.  Concentrations are less than 0.01 mg/L (10 ppb)

              b.  Could include almost any of the remaining elements

B.  SOURCES OF DISSOLVED SUBSTANCES

        1.  RAINWATER

              a.  Low TDS (typically less than 25 mg/L) consisting of
                   the major constituents found in groundwater

              b.  Carbonic acid (H₂CO₃^o) keeps natural pH < 6 and
                   contributes bicarbonate (HCO₃^-) to groundwater 

         2.  LAND SURFACE ACTIVITIES

               a.  Point Sources of Pollution:  landfills, underground
                    storage tanks, septic systems, spills, etc.  (map that
                    reflects point source contamination in Wisconsin)
                         

               b.  Non-point Sources of Pollution:  agriculture, urban
                    runoff, road salt, etc. (map of high nitrate in Wis.)
 

                c.  Groundwater susceptibility to contamination depends
                    on depth to water table and permeability

         3.  SUBSURFACE GEOLOGY

               a.  The influence of rock type (mineral composition)

                    Ohio example:  limestone versus sandstone or sand 

               b.  The influence of  residence time

                    Ohio example: TDS versus well depth  

C.  CASE STUDY:  ARSENIC 

        1.  BACKGROUND

              a.  Definition:  a trace element that is usually found in 
                   sulfide minerals or iron oxide minerals

              b.  Risk:  chronic ingestion can have both toxic and/or
                   cancerous effects (e.g., diabetes, skin cancer)

              c.  MCL: the current drinking water standard for arsenic
                   is 10 ug/L (has a cancer risk of roughly 1:40,000)

 

        2.  ARSENIC IN WISCONSIN

              a.  Most cases of high arsenic concentrations occur in
                   wells that draw water from the St. Peter Sandstone;
                   especially wells drawing from the top of this aquifer,
                   where a sulfide layer is found (arsenic levels in this
                   sulfide layer can exceed 500 ppm).

              b.  Another observation:  arsenic is not initially present 
                   in the wells that later develop this problem. 
                   

              c.  Hypothesis: 

              d. Supporting Evidence: 
               

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