Page 113 - Beaumont Basin Watermaster
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Section 4
Water Quality Conditions
Zone 4 shows the fewest signs of human waste as most homes are served by
consolidated sewer systems.
1A.- Do areas with septic systems have different chemistry than areas with sewers?
The report indicates that there are statistically significant differences between groundwater in
areas with septic systems and groundwater where sewer service is available. The
concentrations of PPCPs, TDS, Nitrate-N, the sum of base cations, Boron, and Isotopes of
Nitrate were all significantly higher in areas with septic systems than in areas with sewer
service.
1B.- Do areas where groundwater recharge with water from the State Water Project or
wastewater treatment plant effluent have different chemistry from other areas?
Strong evidence of nitrate deriving from human waste was detected in Zone 1 as well as
strong biological attenuation of nitrate transported in groundwater.
2.- What sources contribute nitrate to groundwater of the BMZ?
The report indicates that in Zone 1 the isotopes of nitrate values overlap those expected for
human or animal waste. Similarly, in Zone 3 the isotopic composition of water suggest a high
probability of inputs of nitrate from human or animal waste. The presence of PPCPs in most
samples indicates the possibility that septic systems are contaminating groundwater within the
central part of Cherry Valley.
3.- How much nitrate from human waste is making its way into the groundwater of the BMZ?
The report documents the following findings:
Mixing models suggest that between 18 to 30 percent of the nitrate in central Cherry
Valley groundwater is derived from septic systems.
If septic systems were completely phased out, nitrate concentrations in central Cherry
Valley groundwater could decline by 30 percent once a steady state condition is
achieved. The time to reach a steady state is anticipated to be shorter than in other
portions of the BMZ due to relatively high rates of recharge in Zone 3.
Mass balance calculations show that nitrate-nitrogen inputs from septic systems is one
of the largest inputs of nitrogen to groundwater in the BMZ.
If the waste from septic tanks were to be conveyed to the City of Beaumont WWTP,
about 30 percent of the current input of nitrate from human waste to groundwater
would be removed.
Beaumont Basin Watermaster 2018 Annual Report – DRAFT - February 2019 4-7
