EASTSIDE

WATER DISTRICT

Pilot Recharge Basin

RECHARGE BASINS

Most of the land within the District is underlain by highly impermeable strata which forms a barrier, making natural recharge from impoundment created on the surface impracticable.

A concept of excavated Recharge Basins selectively sited and excavated to depths to hydrologically connect to the aquifer (through the hardpan) is currently being investigated. Initial investigations by Boyle Engineering identified areas showing promise as potential recharge sites. The District has retained the services of Kenneth D. Schmidt and Associates, Groundwater Quality Consultants, to conduct the first phase of hydrogeologic evaluation of sites to recharge the aquifer.

Early in 1997, Schmidt and Associates identified a promising location to install a Pilot Recharge Basin. During spring and summer of 1997 the District completed investigations and prepared documents as required by the California Environmental Quality Act.

In the fall of 1997 the District constructed a Pilot Recharge Basin. The Basin is located adjacent to the Turlock Irrigation District (TID) Highline Canal just South of Monte Vista Ave. Dimensions of the Basin are:

Floor Length: 200 feet Area of floor: 0.22 ac-ft

Width: 50 feet Side Slopes: 1 to 1 (45°)

Average Depth: 15 feet Pump Capacity: 500 gpm

As anticipated a clean fine grained sand was encountered at about 13 foot of depth. The District entered into an agreement with TID to purchase and pump water from the Highline Canal.

The District has operated the Basin during the 1998, 1999 and 2000 irrigation seasons. The District purchased water from TID. Water in the Basin is being maintained (with a float switch) between one and two feet of depth, the optimum depth for recharge.

Spring flooding in 1998 delayed initial startup resulting in a short 177 day operating season. The District learned in that first year that as the water warmed and algae formed in the Basin the bottom would seal and the rate of recharge would slow. It was learned as a result of an unplanned shutdown that the algae would die and the recharge rate would recover. In 1999 and 2000, as happened in 1998, when algae began to form, the rate of recharge again began to decline. In order to restore the recharge rate, the District shut the pump off to dry (kill) the algae. In each instance the recharge rate recovered when operations resumed.

Following is a table showing recharge data.

1998 1999 2000

Days 177 231 223

Total recharge (acre-feet) 57.1 114.5 127.7

Total vertical feet (feet) 259 520 580

Average daily recharge (feet/day) 1.5 2.25 2.72

Increase in average daily recharge rate 50% 21%

Start up May 4 Mar 18 Mar 26

Shut down Oct 28 Nov 4 Oct 24

Shown below are two charts, one showing total recharge for 2000, the other comparing total recharge for 1998, 1999 and 2000. Note the flat sections of the curves indicate the pump being shut off to dry the algae.