Sea Water Intrusion in theLos Osos Groundwater Basin

Presentation to the Central Coast RWQCB by the Los Osos Water

Purveyors

Sand SpitDowntown Creek

Upper Aquifer (Zone C)

Lower Aquifer (Zone D)

Lower Aquifer (Zone E)

Bedrock

0

-700

Los Osos Drinking Water• Three water suppliers: LOCSD, GSWC,

and S&T Mutual

• Much of the upper aquifer is high in nitrates, blending or treatment is required

• The lower aquifer currently supplies 77% of the water delivered by these suppliers

• All pumpers are exceeding the yield of the lower aquifer by over 40%

77 %

Lower

Lower AquiferGround WaterElevations - 2001

Morro BayEstuary

CreekValley

At Sea

Lev

el

5’ belo

w

10’ belo

w

Area of Sea Water

Intrusion into Lower

Aquifer

LOCSD

Palisades

Well

OceanSand Spit Downtown

Upper Aquifer (C)

Lower Aquifer (D)

Lower Aquifer (E)

Sea water is the unwanted source in the lower aquifer (values derived

from 2002 Seawater Intrusion study)

910 AFY

560 AFY

420 AFY

Sea Water Intrusion

Upper aquifer leakage

Los Osos Creek

Current actions to combat seawater intrusion

• Purveyors: reduction of water use (pumping) by approximately 15%

• Purveyors: shift in lower aquifer pumping from west to east

• SLO County: retrofit-upon-sale ordinance• Purveyors: attempting to shift to upper

aquifer through blending and nitrate treatment

Results of purveyor actions

• Sea water intrusion estimated in 2002 study:

560 acre feet per year

• Current estimated sea water intrusion:

420 acre feet per year

ISJ Agreement• February 2004 - Basin adjudication initiated by

LOCSD• September 2007 - Water Purveyors and the

County of SLO entered into an Interlocutory Stipulated Judgment (ISJ)

• August 2008 – ISJ approved by court• Current – Conducting technical studies &

negotiating Basin Management Plan (BMP)

BMP Components

• Hydrologic assessment of the basin• Strategy for maximizing reasonable and beneficial

use of the basin– Balancing basin under current development and build-

out– Protect environment influenced by the basin

• Quantification of water rights• Water conservation goals• Well abandonment and construction program• Equitable sharing of costs• Coordination with LOWWP

Current Work Program

• Task 1 – Update steady state hydraulic model – Adjust build-out projections of the basin water demand,

including water conservation– Preliminary evaluation of nitrate treatment unit– Assess the potential of the upper aquifer before and after

the wastewater project

• Task 2– Evaluate and determine creek compartment safe yield– Perform sampling analysis for water quality of creek

compartment

Future Work Program

• Task 3– Prepare Groundwater Basin Infrastructure Master Plan– Evaluate groundwater recharge opportunities– Evaluate options for beneficial reuse of reclaimed water– Prepare a Basin Management Plan

• Task 4– Prepare a transient, dual-density hydraulic model of the

groundwater basin

• Task 5– Annual groundwater monitoring– Transient model updates every 2 years

Los Osos Wastewater Project• Balancing the Los Osos basin will be difficult• The long term sustainability of the groundwater

basin must be a priority for treated effluent produced by the LOWWP– Quantity: Enact cooperative management agreement to

return treated effluent to the groundwater basin– Quality: Enact cooperative management agreement to

address long term management of salt, nutrients and other contaminants

• Potential future hurdles include brine management/disposal and agricultural constraints

Questions?