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Lower Colorado River Hydrology - Groundwater (Colorado River Basins)

Click to view Figure 7.0-4

Figure 7.0-4  Suface Geology of the Lower Colorado River Planning Area

The groundwater basins of the Lower Colorado River Planning Area contain alluvial valleys with significant volumes of groundwater in storage.  As shown in Figure 7.0-4 much of the basin is covered by Quarternary surficial deposits and Holocene to Tertiary alluvial deposits. The basin fill can have very productive water-bearing units.

Basins adjacent to the Colorado River were categorized by Anderson and others (1992) as Colorado River Basins.  Colorado River infiltration was historically the main source of recharge to aquifers in these basins. Other basins in the planning area receive minimal groundwater recharge due to the aridity of the area.  These other basins were categorized by Anderson and others (1992) as West Basins.  The geology of the Colorado River Basins and West Basins are also somewhat different and each are summarized below.  More detailed information on groundwater level changes, water quality, well yields, depth to water, groundwater in storage, groundwater recharge and other groundwater conditions are found in the individual basin sections.

Colorado River Basins

Colorado River Basins include the Parker and Yuma basins.  In these basins the direction and occurrence of groundwater are influenced by the amount of streamflow in the Colorado River, which supplies the largest portion of groundwater recharge. Stream alluvium occurs along the Colorado River and its tributary washes and groundwater in the alluvium is hydraulically connected to the river.

In general, the aquifer consists of recent stream alluvium overlying older, partially consolidated basin-fill deposits, which in turn overlie the Bouse Formation.  The Bouse Formation consists of two zones.  The upper zone is composed of medium to coarse-grained sand which can yield moderate amounts of groundwater under unconfined conditions.  The lower zone contains fine-grained sediments which produce limited amounts of groundwater. Groundwater is found under confined (artesian) conditions in this lower zone. A fanglomerate unit (composed primarily of cemented gravel and thin basalt flows) underlies the Bouse Formation and can yield moderate amounts of groundwater. (Anderson and others, 1992)

Parker Basin

The Parker Basin is composed of three sub-basins; La Posa Plains in the eastern portion, Cibola Valley in the southwest, and Colorado River Indian Reservation in the northwest.

Along the Colorado River groundwater occurs under confined conditions in the Bouse Formation and fanglomerate unit and under unconfined conditions in alluvial deposits.  The recent stream alluvium consists of silt, sand and gravel deposits and groundwater in these deposits is hydraulically connected to the river. In the La Posa Plains sub-basin groundwater is found in relatively small amounts under unconfined conditions.  In this area, groundwater flows toward the Colorado River along stream courses (Figure 7.6-7).  In the Cibola Valley and CRIT sub-basins, groundwater flows parallel to the Colorado River or away from it.

Kofa Mountains -Parker Basin

Kofa Mountains in the Parker Basin

Pre-development groundwater recharge is approximately 241,000 AFA. Estimates of groundwater in storage range from 14 million acre-feet (maf) to 24 maf. The median well yield reported for 75 large diameter (>10 in.) wells was 100 gallons per minute (gpm) (Table 7.6-6).  Water levels declined in most wells measured between 1990-’91 and 2003-’04 (Figure 7.6-7).

Groundwater quality is generally good in the Parker Basin although arsenic, fluoride, nitrate and organic compounds have been measured at concentrations exceeding the Drinking Water Standard in some wells (Table 7.6-7).  Many water quality measurements have been made in the Quartzsite area where septic tanks have caused nitrate contamination of groundwater.

Yuma Basin, Colorado River

Colorado River and canal, Yuma Basin

Yuma Basin

Tertiary and Quaternary basin fill is the primary aquifer in the Yuma Basin.  Thickness of the basin fill may exceed 16,000 feet in some areas but only the upper 2,000 to 2,500 feet is considered hydrologically important because of its excellent transmissive properties.  This aquifer is subdivided into three zones.  In descending order these are the upper fine-grained zone, the coarse-gravel zone and the wedge zone.  The upper zone includes younger alluvium and the uppermost deposits of older alluvium.  Little water is pumped from this zone although beneath irrigated areas, the water table lies within it.  The middle, coarse-gravel zone is the principal water producing unit. Depths to the coarse-gravel zone begin at about 100 feet in the Colorado and Gila River valleys and at about 180 feet below land surface (bls) beneath Yuma Mesa. Throughout most of the Yuma basin the wedge zone underlies the coarse-gravel zone and overlies the Bouse Formation.  The wedge zone is a major water-bearing deposit and consists of interbedded sands, gravel and cobbles.  Depth to the top of this zone is about 160 feet near Laguna Dam and 300 feet in the southern Yuma Valley. (Overby, 1997)  The underlying Bouse Formation is a potential source of groundwater. Units that underlie this formation (marine sedimentary rocks and volcanic rocks) are highly mineralized and deep and are not utilized.

Prior to development, nearly all groundwater recharge was from the Colorado and Gila rivers through direct channel infiltration and annual flooding. The general groundwater flow direction was from the Colorado and Gila Rivers southward under Yuma Mesa. A significant source of groundwater recharge now comes from percolation of excess water applied to crops to reduce salt accumulation in the root-zone.  A groundwater mound has developed under Yuma Mesa as a result of agricultural irrigation and because groundwater flow away from the area is insufficient to drain rising water levels.  This mound and rising groundwater levels in the Yuma area have affected groundwater flow patterns as shown on Figure 7.11-7.  In the western part of the basin, groundwater flow is now generally toward the Colorado River from Imperial Dam to the Northerly International Boundary (NIB).  South of the mound, groundwater flow is still generally south toward the natural drainage, but there also is a component of flow now toward the Colorado River and under the river toward the Mexicali Valley in Mexico (Dickinson and others, 2006).  In the eastern part of the Yuma Basin, groundwater moves from northwest to southeast across the Yuma Desert and exits the basin into Mexico east of the Algodones Fault (Overby, 1997).  The Algodones Fault trends northwest to southeast across the basin south of Yuma and is a barrier to groundwater movement, with higher water levels west of the fault (USBOR, 2009).

Groundwater levels in the basin are also influenced by water management activities.  The “242 Well Field and Lateral” located east of San Luis is a 5-mile wide regulated zone consisting of 35 wells that intercept part of the groundwater flow moving south into Mexico from Yuma Mesa (see Figure 7.0-14).  Irrigation drainage water is a component of this groundwater flow. Water pumped from the well field is delivered to Mexico through the 242 Lateral and other laterals to meet international treaty obligations for Colorado River water deliveries. This activity, as well as groundwater pumping in Mexico, lowers groundwater levels in private wells in the vicinity of the wellfield (USBOR, 2007a).

Pre-development groundwater recharge was approximately 213,000 AFA.  Groundwater storage estimates range from 34 to 49 maf.  The median well yield reported for 327 large diameter (>10 in.) wells is among the highest in the State at 2,456 gpm.  Water levels in wells are generally less than 100 feet bls in most wells measured in 2003-’04 (Figure 7.11-7). As shown in hydrographs of selected wells (Figure 7.11-8), water levels in most wells are relatively stable.

Ground water quality varies across the Yuma Basin with elevated concentrations of total dissolved solids (TDS), arsenic, lead, agricultural pesticides, nitrate and volatile organic compounds in some areas (see Table 7.11-10).   Groundwater was originally more similar in chemical composition to its source waters (Colorado and Gila rivers), but the quality has been altered by more than one hundred years of irrigation activity (Overby, 1997).


water drop  Continue to Section 7.0.2 Hydrology - Groundwater (West Basins)


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