San Simon Sub-Basin - Arizona Department of Environmental Quality

CONCLUSIONS
Groundwater Suitability for Domestic Use
Groundwater in the SS is generally suitable for
domestic and/or municipal use with 67 percent of
sample sites (52 of 77) meeting all health-based
water quality standards. The 25 sites that did not
meet health-based standards are mostly clustered
around the town of San Simon and generally follow
the San Simon River northwest to the sub-basin
boundary. Thus, sites representing large areas of the
SS, particularly in the southern but also in the
western portions, met all health based standards. The
29 sample sites that also met all aesthetics-based
standards also follow the same geographic pattern.
Overall Groundwater Quality
The San Simon sub-basin of the Safford Groundwater
Basin is large and hydrologically complex.
Groundwater quality varies dramatically within the
sub-basin but some general patterns may be
discerned.
Calcium concentrations are best predicted among
anions by sulfate concentrations; in contrast, sodium
concentrations are best predicted almost equally by
sulfate and chloride concentrations with carbonate
concentrations also having a major influence
(multiple regression, p ≤ 0.01). The calcium - sulfate
relationship may be impacted by the dissolution of
calcite and gypsum concentrated by evaporation
during irrigation of agricultural areas that
subsequently recharges the groundwater. 10 In
contrast, the sodium - chloride/sulfate relationship
may be related to the dissolution of evaporite
deposits. 20 28 Geophysical studies and drilling data
indicate that the San Simon Valley contains
significant quantities salt and gypsum deposits, that
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are over 2,700 feet thick at Tanque.
Many constituents significantly decreased with
increasing groundwater and/or well depth
(regression, p ≤ 0.05). However, groundwater and/or
well depth were often unable to be determined.
Groundwater depth data were further complicated by
levels representing artesian or partial artesian flows
rather than water table conditions. Thus, water level
in a well tapping the lower aquifer could be
shallower than a well tapping the upper aquifer. As
such, these correlations are of limited value and
should be cautiously interpreted and used.
The SS is perhaps better described by examining
groundwater evolution from the southern upgradient
areas to northern downgradient areas with Interstate
10 roughly the dividing line.
San Simon sub-basin south of Interstate 10 - The
southern portion of the SS consists of bedrock of the
Chiricahua Mountains, the Dos Cabezas Mountains,
and the unconfined alluvial aquifer which occupies
the valley areas between the Chiricahua Mountains
and the New Mexico border. This area might be
roughly considered all of the sub-basin south of
Interstate 10 with the exception of irrigated areas in
the vicinity of the town of San Simon.
The groundwater in these areas is generally suitable
for domestic and/or municipal use; only three sample
sites exceeded health based water quality standards.
Two sites in the Dos Cabezas had gross alpha
exceedances and a fluoride exceedance occurred near
the New Mexico town of Rodeo. In addition, most
samples also met aesthetics based water quality
standards in these areas. Exceedances occurred with
TDS and sulfate at two sites in the Chiricahuas,
fluoride at two sites in the alluvial aquifer, and at six
sites in the Dos Cabezas involving various
combinations of TDS, chloride, fluoride, iron, and
manganese.
Most sample sites in this area had a calciumbicarbonate
chemistry which often is indicative of
recharge areas. 28 Bicarbonate was the predominant
anion except at two sulfate sites in the Chiricahuas.
This area generally had the freshest groundwater in
the sub-basin with TDS aesthetic based standards of
500 mg/L rarely exceeded and often below 250 mg/L.
The two sites with TDS exceedances in the
Chiricahuas also had sulfate aesthetics based standard
exceedances likely indicating that these sites are
impacted by nearby historic mining operations. 21
TDS concentrations in the Dos Cabezas tended to be
slightly higher than in the Chiricahuas or the alluvial
aquifer.
Constituents, such as arsenic and nitrate, which
commonly exceed health-based water quality
standards in groundwater in Arizona, were low in the
southern portion of the sub-basin. The majority of
fluoride concentrations were also low except in three
locations: the extreme southern portion of the alluvial
aquifer; the northeast portion of the alluvial aquifer
along the New Mexico state line; and the western
portion of the Dos Cabezas range.
San Simon sub-basin north of Interstate 10 - The
northern portion of the SS consists of the upper
aquifer, the lower aquifer, and bedrock of the
Peloncillo Mountains and the Pinaleno Mountains.
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