Evaluation and Reporting on Contaminant Hydrogeological

12-12-01

Conditions at the Tajiguas Landfill

Page 9 of 24

accompanied by tabular data indicating the flow and the mineral quality of the

water from each spring.

Two parallel groundwater monitoring curtains should be placed on either side of

the interceptor trench to determine the nature of contaminated groundwater which

bypasses the trench.

On pages 3.3-18, 3.3-21, and 3.3-22 the draft EIR states,

“A component of the southward groundwater flow is blocked by cross-strike

(east-west trending), low permeability aquitard units. For example, water

level monitoring data indicate that some groundwater flow within the

Vaqueros aquifer is deflected eastward, around the Rincon Formation

(aquitard), where it may discharge as base flow to the alluvium in Arroyo

Quemado (EMCQN, 1994b). This suggests that bedrock aquifers exposed in

Canada de la Pila may be hydraulically connected to those in adjacent

canyons and watersheds via lateral flow along contacts with aquitard units.

These conditions indicate that at least a portion of the groundwater within the

Vaqueros Formation flows eastward and may discharge to the Arroyo

Quemado alluvium.

Seasonally, Vaqueros Formation water levels in monitoring wells e.g.,

(MW-10 and MW-13) near the former Pila Creek channel appear to be at or

above the former ground surface elevation of approximately 250 feet above

msl. This implies that a portion of the groundwater from the Vaqueros

Formation likely discharges to the former Pila Creek channel alluvium or

artificial fill beneath the existing landfill. As of late 1999, the County SWUD

has initiated dewatering and monitoring to minimize this discharge

potential.”

This groundwater flow diversion must be defined in the subsurface in order to

develop a corrective action plan. Such a plan should address methods for

preventing the migration of subsurface contaminants to groundwater and surface

water by investigation and monitoring of the contaminant migratory pathways.

Without a proper understanding of this groundwater flow regime and without

knowing the migratory pathways for contaminated groundwater, minimizing the

discharge potential of contaminated groundwater by strategically placing monitoring

and dewatering wells is very unlikely to be effective and is certainly not verifiable.

Specifically, the groundwater and surface water pathways which can transport

high levels of bacteria from the landfill waste to the beach at the Arroyo Quemada