Heron CRD

1. I could not find an explanation for the submitted hydraulic conductivity of 1550 ft/day.   The average hydraulic conductivity for the test wells was 223 ft/d for the middle well and 2162 ft/d for the west well from the 6-hour pumping tests.  However, only 6 feet (1.5 multiplied by the open interval) was used for aquifer thickness.  The Department uses the entire aquifer thickness which is typically the static to the well bottom or the known water-bearing interval. In this case, there are 13 feet of water-bearing sands and gravels (according to the well log) in the middle well (51 feet static to bottom) and 66 feet of water-bearing interval in west well (68 feet static to bottom).  The hydraulic conductivity using these values for aquifer thickness are 196.6 ft/d for the west well and 103.2 ft/d for the middle well.  These values may be used or more information submitted.
2. Since these wells are over 2000 feet apart, different hydraulic conductivities could be assigned to each area.  Also, most of the drainfields are over a 1000 feet farther inland from the tested wells where conditions may be different.  A pumping test should be performed on a well on or near the southern side of the property away from river.
3. The submitted mixing zone layout is confusing.  The submitted hydraulic gradient measured in four wells is approximately 0.0003 ft/ft at S45°E on the western side of the site, 0.0002 ft/ft at S40°W in the eastern edge, and most of central southeastern part has a gradient approximately south at about 0.00015 ft/ft.  So, the mixing zones should be redrawn (for example, mixing zones on Lots 1 – 4 and 20 – 22 should be roughly south to southeast).
4. Since the hydraulic gradient is very flat, any well pumping activity could affect the direction of ground-water flow.  Drainfields may need to be located a sufficient distance from any well to prevent impacts from the effluent (mixing zone).
5. The submitted cumulative effects were very limited and did not address the effects of the multi-user drainfields on the other drainfields on site (especially with some of the mixing zones drawn in the wrong direction).  Please address this. ADJACENT TO SURFACE WATERS
6. The subdivision is next to the Cabinet Gorge Reservoir on the Clark Fork River.  However, the effect of the dam appears to be to cause the ground water to flow away from the reservoir. Apparently, the reservoir level fluctuates only a couple of feet most of the time.
7. The submitted information did include an analysis of the potential nutrient effects on the reservoir, but used only 46 drainfields and a flow of 395,712,000 ft_/d.  To be conservative, I ran the calculations for the site.  The 7Q10 low flow of the Clark Fork River at the nearest gauge is 3780 ft_/s (326,592,000 ft_/d).  There are 82 lots (four with sanitary exemptions; Lot 75 not mentioned; 8 without Level 2 treatment).  With 8 homes at 50 mg/L and 70 homes at 24 mg/L all @ 26.7 ft_/d:

Nitrate:
    10680 + 44856                                = 0.00017 mg/L < 0.01 mg/L trigger value
2082.6 ft_/d + 326,592,000 ft_/d

Phosphorous:
             22075.56                                = 0.00007 mg/L < 0.001 mg/L trigger value
       2082.6 ft_/d + 326,592,000 ft_/d