Evaluation and Reporting on Contaminant Hydrogeological

12-12-01

Conditions at the Tajiguas Landfill

Page 7 of 24

constructed in natural soils. The out-of-channel basin captures storm water runoff

predominantly from south and west-facing slopes along the east side of Pila Creek

and upstream of the landfill area. Excess runoff from this out-of-channel basin is

captured in the two nearby in-channel basins.

The water from the three basins is routed west of the landfill area in a 48 inch

culvert, which is emptied into the streambed of Pila Creek at a box culvert opening

located approximately 200 feet south of the administrative buildings. The box culvert

opening is identified as the landfill's surface water discharge point. This

surface-water diversion system was not installed until the landfill had been raised

about 25 feet above the streambed.

Surface water runoff from a portion of the landfill upper deck and portions of the

bench areas also drain into this 48-inch culvert. It would seem that surface water

may have come in contact with the landfill mass and will be contaminated. Periodic

monitoring for dissolved chemicals in the water of the diversion system is needed.

The east culvert system collects storm water runoff from the majority of the

landfill proper and routes it to the same discharge point.

The interceptor trench is reported to receive groundwater from underflow from

alluvium and formation rock as well as from water which is collected by the GLCRS.

As of June 4, 1998 (i.e. approximately 6 years of water collection), they have

produced 9,106,943 gallons of water from the trench (i.e. 1,517,824 gallons of water

collected per year). This large volume of water appears to come from two sources;

basal groundwater underflow contaminated by the landfill mass and from leachate

collection pipes located in the upper portions of the landfill.

According to the October 2000 Technical Report Review of Surface Water

Resources Page 6, the Pila Creek watershed yields 46 acre feet per year (i.e.

14,988,125 gallons) and only 1,517,824 gallons of water is collected by the trench

each year. Since Pila Creek is not equipped with a stream gauging station, it is not

known how much surface water run-off is captured by the culvert systems which are

directing the flow into the Pila Creek bed at the surface water discharge point.

Since up to 13 million gallons of water per year potentially bypass the collection

trench, it is important to have an accurate account of the total surface water run-off,

collected by the culvert systems. The balance between the 13 million gallons and the

amount of water, measured at the surface water discharge point, will provide the

volume of groundwater contaminated by the landfill mass, escaping the collection

trench. The water balance must include a determination as to how much water is

used for irrigation, what types of earth materials are undergoing irrigation, all