2012-12-06_REVISION - C1981008 (4)

Home Browse Search

5.3.4 Down - gradient water quality Well GW -18 is located in the overburden immediately south and downgradient of the mined out strata, as shown on Map R -2. The post -mine TDS in the down - gradient well (GW -N18) is higher than the pre -mine value in the same well. In order to better understand the changes in the down - gradient water quality from pre- mining to post- mining, Figure 3 was developed. Figure 3 shows the pH for GW -N18 plotted over time. The data from sampling shows that the pH remained fairly constant at approximately 7.1 but water levels fluctuated significantly in response to the irrigation season. Figure 3 also shows that the pH has decreased slightly, from approximately 7.1 to approximately 7.0 in the post -mine years, but is still very close to neutral. The presence of the calcium carbonate (calcite) in the spoil is buffering the ground water against significant acidization due to pyrite. It is important to understand what is truly different from the pre -mine condition of the well to the post -mine condition where spoil is now present upgradient from well GW -N18. In the pre -mine condition, very good quality water from precipitation infiltrates through the soil through the more permeable strata and picks up dissolved solids. The water quality becomes poor (1000 ppm to over 3000 ppm TDS) as the water moves through the overburden strata. This process takes a long time since the water moves very slowly through the tight strata with low hydraulic permeabilitites, which are in the range of 3.5 to 5.5 ft /day. In the case of the spoil, the hydraulic permeability is increased to 40 ft /day and irrigation water is recharged more rapidly through the permeable, broken spoil material than the pre mine overburden. Water infiltrating into the spoil at the northeastern end of the Phase III reclaimed area spoil may only take 80 days in the spoil to pass through the area. Water passing through the same area in pre -mine conditions would take over 550 days. Figure 4 shows the TDS measurements in GW -N18 plotted over time. In the pre -mine years, the expected seasonal variance can be seen of higher TDS typically in the winter and spring, and lower TDS in the summer and fall as a result of the diluting impact that irrigation water has on the local ground water. There is an increase in the average TDS from pre -mine to post -mine, from roughly 1600 to roughly 2280. This shows that water is permeating through the spoil, oxidizing some pyrite, and then is rapidly buffered by the calcite in the spoil, which keeps the pH consistent while increasing the TDS. This is similar to what happened in GW -N38 as the water level increased. It should be similar, since the chemical reactions are the same. The data in GW -N38 in the post -mine years also shows that some of the spoil water is entering the well, and a large portion is probably saturating the spoil from the bottom of the pit upward. In short, the spoil water chemistry is not significantly different than the in -situ overburden chemistry, the changes simply occur on a faster scale. The net effect on ground water chemistry is a slight decline in quality due to higher TDS, however, the pH is almost unchanged. Over a much longer time, as the spoil water is flushed, the TDS in the spoil should decrease. In the short term, the post mine TDS is not significantly different from pre mine TDS measurements in the area. GW -N44, which is located west of the permit area, has a pre -mine TDS of over 4000 mg /liter. Also, there are no water wells within the vicinity that could be affected by the increase in TDS. SL -14 03DEC12.wpd 70