Evaluation of the Source and Transport of High Nitrate Concentrations in Ground Water, Warren Subbasin, California
By Tracy Nishikawa, Jill N. Densmore, Peter Martin, and
Jonathan Matti
U.S. GEOLOGICAL SURVEY
Water–Resources Investigations Report 03-4009
Sacramento, California 2003
Revised September 18, 2018
Posted June 6, 2003
Prepared in cooperation with the Hi-Desert Water District and the
Mojave Water Agency
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Abstract
Ground water historically has been the sole source of water supply for the Town
of Yucca Valley in the Warren subbasin of the Morongo ground-water basin, California.
An imbalance between ground-water recharge and pumpage caused ground-water levels
in the subbasin to decline by as much as 300 feet from the late 1940s through
1994. In response, the local water district, Hi-Desert Water District, instituted
an artificial recharge program in February 1995 using imported surface water to
replenish the ground water. The artificial recharge program resulted in water-level
recoveries of as much as 250 feet in the vicinity of the recharge ponds between
February 1995 and December 2001; however, nitrate concentrations in some wells
also increased from a background concentration of 10 milligrams per liter to more
than the U.S. Environmental Protection Agency (USEPA) maximum contaminant level
(MCL) of 44 milligrams per liter (10 milligrams per liter as nitrogen).
The objectives of this study were to: (1) evaluate the sources of the high-nitrate
concentrations that occurred after the start of the artificial-recharge program,
(2) develop a ground-water flow and solute-transport model to better understand
the source and transport of nitrates in the aquifer system, and (3) utilize the
calibrated models to evaluate the possible effect of a proposed conjunctive-use
project. These objectives were accomplished by collecting water-level and water-quality
data for the subbasin and assessing changes that have occurred since artificial
recharge began. Collected data were used to calibrate the ground-water flow and
solute-transport models.
Data collected for this study indicate that the areal extent of the water-bearing
deposits is much smaller (about 5.5 square miles versus 19 square miles) than
that of the subbasin. These water-bearing deposits are referred to in this report
as the Warren ground-water basin. Faults separate the ground-water basin into
five hydrogeologic units: the west, the midwest, the mideast, the east and the
northeast hydrogeologic units.
Water-quality analyses indicate that septage from septic tanks is the primary
source of the high-nitrate concentrations measured in the Warren ground-water
basin. Water-quality and stable-isotope data, collected after the start of the
artificial recharge program, indicate that mixing occurs between imported water
and native ground water, with the highest recorded nitrate concentrations in the
midwest and the mideast hydrogeologic units. In general, the timing of the increase
in measured nitrate concentrations in the midwest hydrogeologic unit is directly
related to the distance of the monitoring well from a recharge site, indicating
that the increase in nitrate concentrations is related to the artificial recharge
program. Nitrate-to-chloride and nitrogen-isotope data indicate that septage is
the source of the measured increase in nitrate concentrations in the midwest and
the mideast hydrogeologic units. Samples from four wells in the Warren ground-water
basin were analyzed for caffeine and selected human pharmaceutical products; these
analyses suggest that septage is reaching the water table.
There are two possible conceptual models that explain how high-nitrate septage
reaches the water table: (1) the continued downward migration of septage through
the unsaturated zone to the water table and (2) rising water levels, a result
of the artificial recharge program, entraining septage in the unsaturated zone.
The observations that nitrate concentrations increase in ground-water samples
from wells soon after the start of the artificial recharge program in 1995 and
that the largest increase in nitrate concentrations occur in the midwest and mideast
hydrogeologic units where the largest increase in water levels occur indicate
the validity of the second conceptual model (rising water levels). The potential
nitrate concentration resulting from a water-level rise in the midwest and mideast
hydrogeologic units was estimated using a simple mixing-cell model. The estimated
value is within the range of concentrations measured in samples from wells, further
indicating the validity of the second conceptual model.
A ground-water flow model and a solute-transport model were developed for the
Warren ground-water basin for the period 1956-2001. MODFLOW-96 was used for the
ground-water flow model and MOC3D was used for the solute-transport model. The
model cell size is about 500 feet by 500 feet and the models were discretized
vertically into three layers. The models were calibrated using a trial-and-error
approach using water-level and nitrate-concentration data collected between 1956-2001.
In order to better match the measured data, low fault hydraulic characteristic
values were required, thereby compartmentalizing the ground-water basin. In addition,
it was necessary to parameterize the specific yield distribution for the top model
layer where unconfined ground-water conditions occur into three homogeneous zones.
Separate sets of specific- yield values were needed to simulate the drawdown and
subsequent water-level recovery. In addition, the calibrated natural recharge
was about 83 acre-feet per year. The entrainment of unsaturated-zone septage was
simulated as recharge having an associated nitrate concentration. The volume of
recharge was a function of the measured water-level rise between 1994-98 and the
moisture content of the unsaturated zone. The nitrate concentration of the recharge
water was a weighted function of the assumed nitrate concentration in the infiltrating
water associated with the overlying land use. The simulated hydraulic head and
nitrate concentration results were in good agreement with the measured data indicating
that the mechanism for the increase in nitrate concentrations was rising water
levels entraining high-nitrate septage in the unsaturated-zone.
The calibrated models were used to simulate the possible effects of a planned
conjunctive-use project in the western part of the ground-water basin. The simulated
project included the addition of a new recharge pond and a new extraction well.
In addition, recharge at two existing recharge ponds was increased and three existing
production wells were pumped, treated in a nitrate-removal facility, and used
for water supply. The simulated hydraulic heads increased in the west, the mideast,
and parts of the east hydrogeologic units; however, the simulated hydraulic heads
decreased in the midwest and northeast hydrogeologic units. The simulated nitrate
concentrations increased to above the MCL of 44 milligrams per liter (10 milligrams
per liter as nitrogen) in parts of the west as a result of the increase in simulated
hydraulic head. The simulated nitrate concentrations decreased in part of the
midwest hydrogeologic unit as a result of the artificial recharge and pumping
from the nitrate-removal wells. The simulated nitrate concentrations increased
to above the MCL of 44 milligrams per liter in part of the mideast and parts of
the east hydrogeologic units beneath commercial land-use areas.
Contents
Summary of Major Findings
Ground-Water Quality Highlights
Ground-Water Flow and Solute-Transport Model Highlights
Abstract
Introduction
Purpose and Scope
General Description of Study Area
Multiple-Well Monitoring Sites
Acknowledgments
Geohydrology
Geology
Stratigraphic Units
Depth to Basement Complex
Faults and Ground-Water Barriers
Definition of the Aquifer System
Natural Recharge and Discharge
Ground-Water Development and Artificial Recharge
Ground-Water Levels and Movement
Nitrate in Ground Water
Areal Distribution of Nitrate
Distribution of Nitrates Prior to Artificial Recharge
Distribution of Nitrates After Artificial Recharge Started
1998 Conditions
2001 Conditions
Potential Sources of Nitrate
Natural Soil Nitrate
Nitrates from Septic Tanks
Nitrates from Irrigation-Return Flow
Identification of Nitrate Source
General Chemical Characteristics
Temporal Changes in Nitrate Concentration
Hi-Desert Water District Production Wells
Multiple-Well Monitoring Sites
Nitrate-to-Chloride Ratios
Stable Isotopes of Oxygen and Hydrogen
Background Information
Results
Nitrogen Isotopes
Dissolved Organic Carbon and Fluorescence
Caffeine and Pharmaceutical Analyses
Conceptual Model of Nitrate Transport
Ground-Water Flow and Solute-Transport Models
MODFLOW-96
MOC3D
Model Discretization
Spatial Discretization
Temporal Discretization
Model Boundaries
Subsurface Properties
Ground-Water Flow Properties
Hydraulic Conductivity and Transmissivity
Storage Coefficient and Specific Yield
Vertical Conductance
Faults
Solute-Transport Properties
Model Recharge
Natural Recharge
Artificial Recharge
Model Discharge
Pumpage
Ground-Water Underflow
Model Calibration
Ground-Water Flow Model
Simulated Fluxes
Simulated Hydraulic Heads
Model Fit
Solute-Transport Model
Sensitivity Analysis
Proposed Conjunctive-Use Project
Limitations
Conclusion
References
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Water Resources of California