Slide 12 of 35
Notes:
Wastewater in most septic systems goes through two stages of treatment. It is first clarified in a septic tank which separates out the solids and the water is then drained into the ground in a leach field where natural organisms in the ground are expected to finish the treatment process. Clarified water leaves the tank at the same rate untreated water enters the tank.
Construction of a leach field alters the pre-existing groundwater flow conditions. Water collected from somewhere else, either wells or surface water, is introduced into the ground at a new location. Water originally dispersed over a large area is concentrated into a point source. The water may even have come from a different watershed or recharge area adding more to the total volume of the new location.
Park data indicates that nearly one million people visit Glacier Point during the six months it is open. Assuming that after the long drive, everyone uses the restroom at least once (many use it twice while visiting) and assuming one gallon/flush toilets (most are actually more) then approximately 5,000 gallons of waste water would enter the leach field each day, or 35,000 gallons each week. Rock stability calculations indicate that only 400 to 800 gallons total are needed to trigger the observed slides if it finds it way to rock face fractures.
Also, 100% of wastewater is introduced directly into the ground, unlike snowmelt and storm water.
