WSP01644

• File Number: 8170
• Description: Arkansas Basin Water Quality Issues
• State: CO
• Basin: Arkansas
• Date: 3/14/1997
• Author: Bradford Austin
• Title: Report to the Commissioner of Agriculture - Ground Water Monitoring Activities - Arkansas River Valley Alluvial Aquifer 1994-1995
• Water Supply Pro - Doc Type: Report/Study

The surface water supply has been augmented by diversions to the Arkansas River
drainage basin from the Colorado River basin, Ground water return flows that augment the flow

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and probably represent shallow-lake deposits, The lenses of silt, sand, and gravel were deposited
by braided streams as they aggraded their channels, The materials in the Arkansas River valley
generally are coarser than those in the tributary valleys and contain fewer clay lenses, The
thickness of the alluvium ranges from less than a foot to more than 200 feet in Prowers County,

Alluvium is present in the Arkansas River valley both as Pleistocene and recent terrace
deposits and as recent flood-plain deposits. Terrace deposits form the major portion of the
alluvium in the Arkansas River valley and its tributaries, In the area covered by this study, the
terraces are present throughout the Arkansas river valley on both sides of the river and in all the
major tributary valleys,

Dune sand covers a significant part of the area included in this study, It consists
predominantly of very fine to medium sand and includes some coarse sand, but it also contains
some silt and clay. The thickness of the dune-sand deposits ranges from less than 1 foot to more
than 50 feet; where the dunes are actively migrating the thickness may differ considerably in a few
years,

HYDROGEOLClGY

The alluvium contains the major available supply of ground water in the area covered by
this study, Throughout the Arkansas River valley and its tributary valleys, these deposits form an
almost continuous unconfined aquifer that is in hydraulic connection with the Arkansas River.
Because ofitshigh permeability, the alluvium yields large quantities of water to wells in many
parts of the area, Most of the irrigation wells obtain their entire yield from the alluvium,
Supplemented by water supplied through trans-mountain diversions, the river and alluvial aquifer
supply all of the water used for irrigation and a significant portion ofthe domestic supply for the
area,

In general the areas of dune-sand deposits are good infiltration areas for recharge to the
underlying alluvial material, However, the few wells in dune-sand areas yield only small quantities
of water suitable for only domestic or stock wells, In areas where small saturated thickness of
dune sand is underlain by impervious material, a few wells have been drilled through the sand into
the impervious material, thus providing a small reservoir for the accumulation of water.

HYDROLOGY

In the Arkansas River valley, surface water and ground water are two components of one
hydraulic system, The valley-fill aquifer is recharged by precipitation, applied irrigation water,
and leakage from canals and reservoirs. Recharge to the aquifer from irrigated land is from 45 to
50 percent of the applied irrigation water and precipitation, Ground water withdrawals lower the
water table and tend to reduce flow towards the river and, locally, under heavy pump age, may
temporarily lower the water table sufficiently to reverse the ground water gradient and cause
water to flow from the stream to the aquifer,