Managed aquifer recharge as a tool for sustainable management of groundwaterg gquantity and quality in agricultural basins.

C M Schmidt1* A T Fisher1 A Racz1 C G Wheat2 B Lockwood3C.M. Schmidt1 , A.T. Fisher1, A. Racz1 C.G. Wheat2, B. Lockwood3

1Department of Earth and Planetary Sciences, University of California, Santa Cruz

Toward Sustainable Groundwater in Agriculture Conference June 16, 2010

Presentation OutlinePresentation Outline

• Introduction to Harkins Slough MARIntroduction to Harkins Slough MAR system

• Dynamics of infiltration• Dynamics of infiltration• Nutrient removal during infiltration• Isotopic evidence for denitrification• Conclusions and Implicationsp

Pajaro Valley Ground Water Basin

• Water use is primarily• Water use is primarily agricultural

G d t t ti• Ground water extraction is 60-70k ac-ft/year

• Basin is severely overdrafted

• Seawater intrusion is a significant problem in the

t lcoastal zone.

Harkins Slough Managed Aquifer Recharge Project

Harkins SloughRecharge Pond

Development of local ground water mound

(m-m

sl)

elev

atio

n (

Wat

er ta

ble

Period of MAR

W

2008 Water Year3• ~1 million m3 are diverted for MAR per year

(~810 acre-feet)

• Infiltration rates range between 1000- 20,000 m3/d(0.8 – 16 acre-ft/day)

MAR reduces nitrate concentration in the perched aquifer

m-mWater TableSite Map N m

sl

NO3-

MW 3

NO

3-(m

M)

MW 2

200 m

Research Objectives1 What is the load of nitrogen delivered to the aquifer1. What is the load of nitrogen delivered to the aquifer

through MAR?

2. Is there a reduction in the nitrogen load during infiltration?

3. What is the mechanism of nutrient removal during MAR?

4. What are the optimal conditions for load reduction in terms of physical hydrology (infiltration rate) and fluid chemistry (nitrate concentration, carbon sources) ?

Recharge through the unsaturated zone

Diverted wetland waterRecharge Pond

Preferred Paths Unsaturated Zone

50 m

Saturated ZonePerched Aquifer

confining clay layer

Regional Aquifer

Infiltration above an inverted water table

Pond

Unsaturated Zone

50 m1 m

Saturated Zone

InfiltrationInverted water table

1 m

confining clay layer

Recharge

Pond InstrumentationPiezometer Nest

Profile

Fluid sampling lines

12

3

Piezometer for thermal

measurements

Piezometers for Fluid sampling

North

-0.5 m4

North

-1.0 m

meters0 100

Piezometer NestPiezometer Nest

Lysimter Nest

Pond InstrumentationPiezometer Nest

Fluid sampling lines

Piezometer for thermal

measurements

Piezometers for Fluid sampling

-0.5 m

-1.0 m

Th l t d• Thermal measurements are used to determine infiltration rates at each instrument nest.

• Fluid samples collected weekly from pond and piezometers

Profile

The focus of infiltration moves across the pond

12

34

Profilemoves across the pond

d)

Pond AverageProfile 1Profile 2Profile 3

n ra

te(m

/d Profile 3Profile 4

••Whole pond infiltration

nfilt

ratio

n Whole pond infiltration decreases from ~1 m/d to 0.1 m/d after about 10 weeks.

In

•While infiltration rate decreases in some locations, in other locations infiltration rate

Water Year 2008

increases

A. Racz 2008

Profile

The focus of infiltration moves across the pond

12

34

Profilemoves across the pond

d)

Pond AverageProfile 1Profile 2Profile 3

n ra

te(m

/d Profile 3Profile 4

••Whole pond infiltration

nfilt

ratio

n Whole pond infiltration decreases from ~1 m/d to 0.1 m/d after about 10 weeks.

In

•While infiltration rate decreases in some locations, in other locations infiltration rate

Water Year 2008

increases

A. Racz 2008

Nutrient Load Reduction During Infiltration

1Profile

Diverted waterProfile 1Profile 2μM

)

12

34

Profile 3Profile 4

Nitr

ate

(μ(k

g)

Load Reduction • Nitrate concentration is consistently reduced during i filt ti

Nitr

ate-

N infiltration

• Load reduction of >50%,~600 kg of nitrate-N

Water Year 2008

Denitrification is one potential removal mechanism

• In low oxygen• In low oxygen environments, microbes can utilize nitrate to ut e t ate tooxidize organic carbon.

• During respiratory denitrification, nitrate is reduced t N hi hto N2 gas which can leave the system.

Isotopic Signature of Denitrification

δ (‰) = -1)( x 1000δ (‰) = 1)( 000

NO

‰)

NO3-

fertilizer

of N

O3

(‰

Soil NO3

-

δ18 O

NitrifiedNH4

+ fertilizer

Kendall et al., 2007δ15N of NO3 (‰)

Isotopic Signature of Denitrification

δ (‰) = -1)( x 1000δ (‰) = 1)( 000

NO D it ifi ti

‰)

NO3-

fertilizer

Denitrification causes an increase in

both δ15N and δ18O ofthe residual NO3

of N

O3

(‰

Soil NO3

-

3

δ18 O

NitrifiedNH4

+ fertilizer

Kendall et al., 2007δ15N of NO3 (‰)

Is denitrification the mechanism of nitrate removal during infiltration?

NO3-

nitrate removal during infiltration?

O3

(‰)

fertilizer

O o

f NO

Soil NO3-

δ18 Nitrified

NH4+ fertilizer

Diverted waterPiezometers

δ15N of NO3 (‰)

Nitrate removal efficiency is greatest at infiltration rates between 0.2- 0.5 m/d

Infiltration rate% NO d

PZ-2PZ-1

ved In

% NO3- removed

3-re

mov

nfiltratio

t of N

O3

on Rate PZ-3 PZ-4

Per

cent m

/d

PZ 3 PZ 4

2008 Water Year

Rapid Denitrification During Infiltration

d-1 )

(μm

ol L

-1rif

icat

ion

of D

enitr

mum

Rat

eM

axim

After Green et al., 2006

Rapid Denitrification during Infiltration

d-1 )

(μm

ol L

-1rif

icat

ion

of D

enitr

mum

Rat

eM

axim

After Green et al., 2006

Conclusions

• The focus of rapid infiltration (as high as 3 m/d) moves across the pond.

• ≥50% of the nitrogen load is removed during 50% o e oge oad s e o ed du gshallow infiltration

• Analysis of δ15N and δ18O of nitrate suggests• Analysis of δ15N and δ18O of nitrate suggests removal of nitrogen by microbial denitrification.

• High rates of denitrification are maintained over a broad range of conditions in this system.

Could small, seasonal, distributed artificial recharge projectsplay a significant role in basin management?p y g g

Advantages of Distributed MARg

• Minimal infrastructure needed

• No new large water supply needed

P t ti l t f t i t BMP• Potential component of nutrient BMP (infiltration ~ as effective at load reduction as vegetated buffer strips)

• Local control and community participation in resource managementmanagement

Could small, seasonal, distributed artificial recharge projectsplay a significant role in basin management?p y g g

Total Ag Area 6 400 acres

1. How much recharge is possible? Total Ag.  Area 6,400 acres

Dedicate 0.5 % to MAR  320 acres

Recharge Efficiency 20 AFY per acre~ 30% of Annual

Total Recharge 6,400 AFY

2. Is it economically feasible?

30% of AnnualBudget Imbalance

Recharge Basin Area 1 acre

Drainage Area 100 acres

y

Capture 0.2 ft runoff/year 20 AFY

Current Pumping  Fee  $80 per AF

MAR “ l ” $ 1 600

Strawberry Profits:~$1 900 per acreMAR “value” $ 1, 600 per 

acre

$1,900 per acre(PANNA.org)

Questions?

Thank You,

PVWMA, Carol Kendall, Adina Paytan, Marc Los Huertos, Jessica Sharkey, Joanna Hoffman, Tess Russo, Nick Massetani, Dyke Andreasen.

Denitrification is not carbon limited in this systemy

OC available in diverted wetland water

OC

uM

D

OC required to reduce all nitrate in diverted water

Water Year 2008

Reducing conditions can develop in the shallow subsurfaceduring rapid infiltration of oxic water

2 2DO NO3- Mn 2+ Fe 2+

GS

(cm

)D

epth

BG

Concentration in pore water (μM)