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Hydrology and Nitrate Loading

 While the ground surface is very dry during the summer and fall, the water table is only about five feet below the surface through most of the ranch.  The Geotechnical Report for the Pond Area states, “the water table tends to be reasonably stable at an elevation about 5 feet below the meadow elevation adjacent (to) the west side of the pond. This water table is likely to vary seasonally and in a wet year could approach the surface. In an extended dry period the water level could fall a bit but we suspect that 5 feet below meadow elevation is nearly as low at the water table gets. Area groundwater moves most efficiently through a sandy lens that tends to be 5 to 6 feet below grade although saturated soils prevail to substantial depths (probably many hundreds of feet).”

Soils reports for several lots on the east side of the ranch indicate a water table of from 4.75 feet to 10 feet below the ground surface.  The variation in depth may be due more to variations in the ground surface elevation than to variations in the level of the water.

Measurements by the ranch foreman in 2007 and 2008 show that the water table in the meadow west of the pond is relatively stable throughout the year, though it does rise when rain and snow begin in November.  The ground water level, at least in the meadow area, does not drop when the water level in the river drops in October.  See the Ground Water Level Measurements in the Appendices.  When a sufficient volume of snow melts in the spring, substantial surface water appears on the river’s floodplain.  There is also a low area east of Hashknife Road, across from Lots 12 and 13, that holds water when the snow melts.

Nitrate Loading

In 2007, a U.S. Geographic Service study found that nitrate levels in the area's ground water aquifer are increasing due to contamination from residential septic systems. The contamination has public health implications because ground water is the sole source of drinking water for many area residents. 

The U.S. Environmental Protection Agency has established 10 parts per million (ppm) of nitrogen as the maximum allowable nitrate concentration in drinking water for public water supply systems. Oregon law sets a nitrate concentration of 7 ppm as the level at which regulatory action must be taken.  While only 10% of 200 area wells surveyed have a nitrate concentration of over 4 ppm in 2007, the study concluded that contamination will eventually reach unacceptable levels if the amount of nitrates put into the ground every year (nitrate loading) is not decreased from present levels.  Deschutes County is working on programs to require residents to equip septic tanks to remove nitrates. 

From a natural resource point of view, the concern with excessive nitrates is that they could support the growth of algae and weeds in waterways.  But plant growth is almost always limited by a scarce supply of phosphorus, not nitrogen or nitrates.  So if nitrates increase in the river, it is most likely that weed and algae growth will not increase.  The Little Deschutes River, and even its backwaters, do not have a significant algae or weed problem today.  A water sample from the pond, fed by groundwater and well water, found no detectable nitrate at all.

For more information about nitrate loading, see the following two documents:

Questions and Answers About the Effects of Septic Systems on Water Quality in the La Pine Area, Oregon

Evaluation of Approaches for Managing Nitrate Loading from On-Site Wastewater Systems near La Pine, Oregon Scientific Investigations   

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