Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
Geophysical logs in a borehole about 1 mile northwest of Powerline FRS show very low
resistivities indicating significant clay content. Based on the amount of subsidence that has
occurred in this area, highly compressible clays are anticipated to be present in this area.
The character of the geophysical logs for wells located near, or to the north and west of, the
Powerline FRS indicated the presence of zones of significant thickness with resistivities less than
about 10 ohm-m. Therefore, subsidence that results from consolidation of clay-rich materials
occurs relatively slowly. These areas are shown by InSAR to have been actively subsiding
during the 1990s in spite of relatively constant regional groundwater levels.
Historical deep wells were completed near the CAP alignment at or to the south of the Vineyard
Road FRS. The character of the geophysical logs for these wells is consistent with the published
descriptions of the upper alluvial unit (UAU), and very different from wells completed within the
middle fine-grained unit (MFGU) northwest of the Powerline FRS. Between depths of 600 and
1,200 feet bgs, at least 16 zones of higher resistivity (greater than 10 ohm-m), typically at least a
few feet thick, could be identified. These higher-resistivity zones were interpreted to contain
more granular, higher-permeability materials, rather than clayey material. This alluvial basin
section was thus interpreted to include significant interbedded or interfingered units, fluctuating
between sandy/gravelly horizons and clay-enriched horizons. With sufficient interconnection of
the higher-resistivity, higher-permeability horizons, the basin alluvium could have been drained
more rapidly as the groundwater level dropped, and subsidence would have occurred much faster
than in the clay-rich alluvium to the north. Geophysical logs indicate that the basin alluvium
south of the Vineyard Road FRS appears to be relatively clay poor compared to basin alluvium
to the northwest of the Vineyard Road FRS. Below depths of about 1,200 feet bgs the
resistivities increased and rarely fell below 10 ohm-m.
Hydrogeologic Conditions
A reasonable estimate of predevelopment groundwater levels can be made from well data
indicating a groundwater elevation of about 1,325 feet above mean sea level (amsl),
corresponding to a depth to water of approximately 275 feet, in the late 1930s to early 1940s.
In 1978, groundwater levels at the northern and southern ends of the study area had dropped
over 200 feet from the estimated predevelopment level, corresponding to a depth to water of
about 500 feet bgs or an elevation about 1,100 feet amsl. In the middle of the study area,
groundwater levels had dropped less, and were up to about 70 feet higher than in the surrounding
area. Suburban growth in the area north of the Powerline FRS, and the establishment of major
agriculture to the southwest and west of the Rittenhouse FRS, were probable reasons for the
development of major pumping centers in 1978.
By 2009, the water level trend across the PVR study area had changed significantly. Continued
suburban development in the northern part of the study area had resulted in continued pumping
of groundwater with a further groundwater level decline such that the depth to water was nearly
600 feet bgs. Replacement of pumped groundwater by CAP water delivery for agricultural use
resulted in significantly reduced pumping and recovery of groundwater levels by as much as 80
feet from the 1980s to the present. Groundwater levels remained stable or declined moderately
(perhaps up to about 40 feet of decline from the 1970s to the present) through the Vineyard Road
FRS section of the study area.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 43