MPRWA DEIR Review 2 Subsurface Intakes

8/21/2019 MPRWA DEIR Review 2 Subsurface Intakes

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Review of Subsurface Seawater Intakes

DEIR, Monterey Desal Project

DRAFT Presentation to Monterey

Peninsula Regional Water Authority

23 June 2015


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Outline

Overview of Project and Proposed Subsurface Seawater

Intakes (SSIs)

Overview of Hydrogeologic Setting

Site-Specific Feasibility Evaluation of SSIs

− Borings

− Pumping Test

− Groundwater Models

Findings

Discussion of Assumptions and Uncertainties

Conclusions


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Proposed Production and Required Feedwater

Option 1 (Proposed Project)Production: 9.6 mgd (10,754 AFY = 6,667 gpm)

Feedwater: 24.1 mgd (26,997 AFY = 16,736 gpm)

from 10 Slant Wells at CMEX Site

Option 2 (Project Variant)

Production: 6.4 mgd (7,169 AFY = 4,444 gpm)

Feedwater: 15.5 mgd (17,363 AFY = 10,764 gpm)

from 7 Slant Wells at CMEX Site

Intent is to maximize contribution to the intakes from

the ocean and minimize contribution from inland

coastal aquifers.


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Schematic Example of a Slant Welleosyntec )

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Proposed Slant Well Locations at CMEX Siteeosyntec )

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Coastal Aquifer Hydrostratigraphic Units

(Figure 4.4-2 of DEIR)

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Groundwater Elevations

in Salinas Valley 180-ft Aquifer


Groundwater levels

well below sea level

several miles inland

due to overdraft of

aquifers.

MAKE LABELS

LARGER


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Extent of Seawater Intrusion



Chloride > 500 mg/L

extends 8 miles

from the coast

(2013).

The SVGB is

hydrologically

connected to the

Monterey Bay by

ocean outcrops of

the 180-Foot and

400-Foot Aquifers

that outcrop a few

miles offshore.

.


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Groundwater Elevations



Groundwater levels

well below sea level

several miles inland.

MAKE LABELS

LARGER


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Extent of Seawater Intrusion



Chloride > 500 mg/L

extends 3.5 miles

from the coast

(2013).

.


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Feasibility Evaluation and Design of

Subsurface Intakes

Site-Specific Properties and Testing

6 exploratory borings− water quality samples in 15 aquifer zone locations

−grain-size analysis and calcs of hydraulic conductivity

− laboratory testing of Kh and Kv on core samples

− geophysical logs

7 monitoring well clusters

Long-term pumping test of first slant well currently in

progress.


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Geologic Cross Section of CMEX Area


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Feasibility Evaluation and Design of

Subsurface Intakes

Groundwater flow modeling of slant well Intakes at coastal

margin provides predictions of

Portions of contribution to subsurface intakes from the

ocean and from inland

Potential influence of the pumping on coastal margin

aquifers

−

drawdown of groundwater levels− change in groundwater flow

− change sea water intrusion rates


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Groundwater Modeling

Three groundwater models of different scale

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Groundwater Models of Three Scales

Regional Salinas Valley Integrated Groundwater and

Surface Water Model (SVIGSM)

North Marina Groundwater Model (NMGWM, Geoscience2013-2015)

CMEX Model (CM, Geoscience, 2014)


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Salinas Valley IGSM

Regional Salinas Valley Integrated Groundwater and

Surface Water Model (SVIGSM)

− Entire Salinas Valley Basin Management Tool

(Montgomery Watson, 1994; WRIME, 2008)− 650 sq mi model domain with ~0.4 sq mile cell size

− 3 Layers: 180-ft, 400-ft, 900-ft aquifers.


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North Marina GWM

North Marina Groundwater Model (NMGWM, Geoscience

2013-2015)

− MODFLOW, MT3D, SEAWAT More detailed designed as

tool to evaluate feasibil ity and potential impact of SSIs

− coverage of 149 sq mi area along coast; Offshore to 5 mi

and 5+ mi inland; 200 x 200 ft cell size.

− 8 Layers: Benthic Zone, Dune Sand, 180-ft and 180 ft equiv,

400-ft, 900-ft aquifers and intervening aquitards.

−Uses results of SVIGSM as boundary conditions.

− Transient runs with variable climate conditions for 63 years.


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CMEX MODEL

CMEX Model (CM, Geoscience, 2014)− SEWAT. More detailed tool to evaluate feasibility of SSI in

CMEX area.

− 4 sq mile domain. 20 x 20 ft cell size. Offshore to 4800 ft

and ~6000 ft inland− 12 layers:

Benthic Zone (constant sea level),

Dune Sand, 180-ft and 180 ft equiv,

400-ft, 900-ft aquifers and intervening aquitards.

− Uses results of NMGWM for boundary conditions.

− Will be calibrated to long-term slant well pumping test

− Then NMGWM updated


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Model Drawdown in Dune Sand Aquifer

Model-calculated

maximum inland

extent of 1 ft

lowering

(drawdown) ofgroundwater due

to project

pumping is ~5

miles.

(1 ft contours of

drawdown)


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Model Drawdown in 180-ft Aquifer

Model-calculatedinland extent of

1 ft lowering

(drawdown) of

groundwater dueto project pumping

is ~7 miles.

(1 ft contours ofdrawdown)


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Model Drawdown in 180-ft Aquifer

and Locations of Wells


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Drawdown Impact is Less Than Signif icant

No local wells within the area of influence would beadversely impacted by the drawdown caused by project

pumping.

− Pumps and screens are deeper than the predicted drawdown,

−

Shallow wells no longer used, or− Wells are screened in a deeper aquifer with limited hydraulic

connection to the Dune Sands or the 180-Foot Equivalent

Aquifers.

− The nearest municipal water supply wells (Marina Wells 10,

11, and 12) are more than 2 miles to the SE and screened inthe 900-Foot Aquifer.

Consequently, the DIER concludes that impact of the

project on neighboring, local groundwater wells is less than

significant.


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Proportion of Ocean Water and Freshwater

Proportion of Ocean Water (X) and Inland Fresh Water

(1-X) calculated from model chloride content of intake

water (Feedwater Salinity FS).

− Ocean Water Salinity (OWS) Chloride Conc = 33,500 mg/L

− Inland Groundwater Salinity (IS) Chloride Conc = 440 mg/L

For example Intake Salinity of

32,000 mg/L consists of 95%

Ocean Water and 5% InlandGroundwater.

X * OWS + (1-X) IS = FS

X * OWS + IS - X*IS = FS

X*OWS - X*IS = FS - IS

X(OWS-IS) = FS - IS

X = (FS- IS)/(OWS-IS)


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Inland Groundwater Take is Replenished

Based on NMGWM pumping simulations, the intake

water consists of

− 94.5% ocean water, and

− 5.5% inland groundwater which is 1,458 AFY.

The project will return fresh water, which is equal to the

portion of inland groundwater pumped, to the Salinas

Groundwater Basin via the Castroville Seawater Intrusion

Project (CSIP) ponds.

Thus, the DEIR concludes the project will result in no

net depletion of inland groundwater.


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Potential Influence on Contaminant Plumes

at Fort Ord

DEIR Fig 4.4-18 Drawdown in180 and Ft Ord Plumes


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Potential Influence on Contaminant Plumes

at Fort Ord

OU1 TCE A-Aquifer Plume (TCE plume 2.25 mi SE):

Less than significant because drawdown of project

pumping much less than local remedial pumping

OUCTP A-Aquifer Plume (carbon tetrachloride plume 2

mi SE): bioremediation in progress. Cal Am monitoring

and mitigation if needed.

OUCTP Upper 180 ft Aquifer Plume (3 mi SE). Less

than significant because drawdown of project pumping

much less than local remedial pumping


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The Project Reduces Inland Extent of

Sea Water Intrusion

Project pumping accelerates seawater intrusion to the

SSIs in the CEMEX area, but reduces the rate of sea

water intrusion further inland.

Project coastal margin pumping locally reverses existinginland flow of groundwater and draws some inland

groundwater toward the coast.

Thus the project decreases seawater intrusion to

inland aquifers.


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The Project Reduces Inland Extent of

Sea Water Intrusion

DEIR Fig 4.4-16

Exist ing Condit ions (No Project) 24.1 MGD Pumping

Inland flow direction in Coastal Aquifers Local Reversal of Flow toward Ocean

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Conservative Model Assumptions

Offshore No-Flow Boundary Conditions

Offshore portion of Model Layer 1 (“Benthic Layer”) is

constant sea level elevation and salinity, but the offshore

boundary conditions for all the other layers are no flow

boundaries rather than constant sea level elevation.

The large offshore extent of the model layers provides a

“reservoir” of “sea-water groundwater” beneath the sea

floor, but no flow boundaries result in conservatively large

model contribution of inland flow to pumping beneath thecoastal margin.


30/36


Conservatively Low Kh between Dune sand and 180-FTE Aquifers at Coastline

Borings at the CMEX site show that low permeability clay

layers between Dune Sand and 180-FTE Aquifers, which

are present inland, do not extent offshore.However, the model includes a relatively low permeability

layer (Model Layer 3, Kh = 5 ft/d) between the Dune Sand

and 180 FTE Aquifers extending offshore.

Thus, the model may underestimate the hydraulicconnection between the 180-FTE Aquifer with the the

Dune Sand Aquifer and the Ocean.


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ConservativelyLow Kh (5 ft/d)

between Dune

sand and 180-FTE

Aquifers atCoastline

(Model Layer 3)

P t ti ll N C ti H d li


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Potentially Non-Conservative Hydraulic

Conductivity Values in Models

Horizontal Hydraulic Conductivity (Kh) values (340 and 114ft/d) assigned to the Dune Sand and 180 FTE Aquifers at

coastal margin and offshore may be optimistically high.

Vertical Hydraulic Conductivity (Kv) value (10 ft/d) assigned

to some of the Dune Sand aquifer may be optimistically high.

Thus, the models may overestimate hydraulic connection

between the 180-FTE and Dune Sand Aquifers with the

Ocean.

May be appropriate to update the NMGWM properties

based on calibration of CMEX model to long-term

pumping test of Slant Well.

P t ti ll N C ti H d li


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Potentially Non-Conservative Hydraulic

Conductivity Values in Models

Hi Kv (10 ft/d) for

Dune Sand Aquifer (Model

Layer 2) may

overestimate

hydraulicconnection to the

Ocean

P t ti ll N C ti L ti f


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Potentially Non-Conservative Location of

Slant Wells in Models

The models represent the slant wells extending wellbeneath the sea floor, but the actual slant wells may not

reach that far.

Test slant well barely reaches the ocean margin.

At the ocean margin the well screen is more the 200 ft

beneath sea level.

Thus, the models may overestimate hydraulic connection

between the slant well intakes and the Ocean.May be appropriate to update the models with the

actual Slant Well locations, or conduct sensitivity

model runs.

Substantial Separation between Slant Well


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Substantial Separation between Slant Well

Intakes and the Ocean Floor

Actual geometry of installed slant well.

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Conclusions Regarding Project Intakes

The models provide reasonable simulations of pumpingfrom the subsurface intakes.

The contribution of inland fresh groundwater to the

proposed pumping beneath the coastal margin is minor and

can realistically be returned to the Salinas Basin.

The potential impact to inland wells is not significant.

The project pumping would decrease sea water intrusion to

inland aquifers.Updates to the model predictions can be made based on

the long-term pump testing currently in progress at the

slant well.

MPRWA DEIR Review 2 Subsurface Intakes

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