Formation and Dissipation of Trihalomethanes during Aquifer Storage and Recovery Operations

Jason PulleyCity of Salem Public Works

System Overview

• Slow sand filtration– 80 MGD firm treatment capacity– 66 MGD transmission capacity

• 183,000 service population• 30 MGD average• 55 MGD peak day• 136 MG system storage

ASR Operations

• ASR used to supplement treatment plant production during high-use periods– Used in winter during high turbidity events

• Injection typically from Nov-March– Inject at two of four wells ≈ 3.5 mgd

• Storage goal of 500 mg– Recovery from 3 of 4 wells ≈ 6.7 mgd– 100% recovery with use of groundwater rights

Well Summary

Name Completion Date

Completed Depth (bgs)

Casing Depth (bgs)

SWL (bgs)

Flow (gpm)

ASR 1 3/95 330 280 197 1,000

ASR 2 1/96 330 280 195 1,750

ASR 4 1/98 327 272 175 1,800

ASR 5 9/97 350 281 186 1,450

Distribution System Water Quality (Typical)

pH 7.2TOC 0.64 mg/LTDS 30.0 mg/LAlkalinity 20.4 mg/LCalcium 3.64 mg/LMagnesium 0.95 mg/lPotassium 0.57 mg/LTTHM 17-57 ug/L

Production History

0

100

200

300

400

500

600

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Injected Recovered

Mill

ion

Gal

lons

56 mg native groundwater

2,571 mg injected2,245 mg recovered

Observations• Elevated THM concentrations noticed in

2005– Isolated at ASR 5– Quickly dissipated during recovery operations

• Observed again in 2006 at ASR 5 and ASR 4– Possibly related to storage volume– 350 mg in 2005; 400 mg in 2006– Had only stored 350 mg once (2000) and never

400 mg– Historic levels had fluctuated but never at the

levels observed

Previous Studies• Singer et al. (1993) J. AWWA

– THMs and HAAs are removed from chlorinated water during storage

– Precursors are also removed to a significant degree– Biological mechanisms are suspected

• McQuarrie et al. (2003) J. Env. Eng.– Acquifer storage of chlorinated water resulted in a 44%

reduction in THM formation– THM removal accelerated under anoxic conditions– Significant DOC reduction during aquifer storage

Previous Studies

• Pyne et al. (1996) AWWARF – Focused on five sites with injected treated

drinking water– Storage periods from 36-127 d– THM reductions of 25-100%– Some loss attributed to dilution/mixing;

biodegradation plays a significant role– Also reported reduction in THM precursors

Previous Studies• Landmeyer et al. (2000) J. AWRA

– Las Vegas Valley Water District ASR– Observed increases in THM concentrations during

recovery– Conc. decreased with continued pumping

• Adsorption• Mixing• Microbial degradation

– Lab studies show no significant CHCl3 biodegradation (aerobic or anaerobic)

• Low organic carbon content restricts microbial attenuation– CHCL3 entrained in water or formed in situ will tend to

persist

Initial Investigations• Monitor THM concentrations over a 30-day storage

period– Weekly measurements of THM at each of four wells

• Collect samples from each well and finished water from TP– 7-d THMFP, DOC, SUVA on all samples– 30-d THMFP on finished water– 7 & 30-d SDS on finished water

• Time series analysis after 30-d storage (every 10-min for 1 h)

Analytical Methodology• EPA Method 524.2

– Purgeable organic compounds by capillary column GC/MS

– Alternate methods 551.1 & 552.2 (liquid extraction with ECD)

– All provide full speciation• Hach Procedure 10132

– Colorimetric read on spectrophotometer– All results reported as chloroform (CHCl3)– Estimated detection limit of 6 g L-1

Hach Procedure 10132

• Provides “screening level” data– Low-cost quantitative data– Internal comparisons within +/- 10%– Prep and analysis time < 30 min

• Hach validation– vs 524.2, 551.1, 552.2– R2 values of 0.906, 0.938, and 0.959,

respectively

NOM & SUVA• NOM – a mixture of humic and nonhumic organic

substances– Contributes to DBP precursor levels and speciation

• Humic substances have higher SUVA and formation potential than nonhumic– SUVA = UV @ 254 / DOC

• SUVA provides an indicator for DBP formation• SUVA > 2 L/mg-m generally considered high

formation potential

ASR THM Formation Potential

7d THMFP DOC SUVA FCl*---g/L--- --mg/L-- --L/mg-m-- ---mg/L---

ASR 1 82 0.95 2.9 4.2

ASR 2 90 0.85 3.5 3.6

ASR 4 87 0.65 3.1 4.6

ASR 5 95 0.58 2.6 4.7* Initial chlorine dose of 6.0 mg/L

Treatment Plant Effluent

DOC: 0.67mg/L SUVA: 1.9 L/mg-m7d FP: 41 g/L 30d FP: 36 g/L7d SDS: 41 g/L 30d SDS: 62 g/L

Historic Storage Period THM Concentrations

1998 2000 2002 2004 2006 2008

Tota

l Sto

red

Vol

ume

(mg)

0

100

200

300

400

500

600

THM

Con

cent

ratio

n (u

g/L)

0

20

40

60

80

100

120

140

Storage VolumeTHM

Date1/1/2006 5/1/2006 9/1/2006 1/1/2007 5/1/2007 9/1/2007 1/1/2008

Cum

ulat

ive

Vol

ume

(mg)

0

100

200

300

400

500

600

Con

cent

ratio

n (u

g/L)

0

20

40

60

80

100

120

140

Cumulative Volume THM Concentration

THM and Storage Volume

Date

1/1/06 2/1/06 3/1/06 4/1/06 5/1/06 6/1/06 7/1/06 8/1/06 9/1/06 10/1/06 11/1/06 12/1/06

Cum

ulat

ive

ASR

Sto

rage

(MG

)

-50

0

50

100

150

200

250

300

350

400

450

500

Con

cent

ratio

n (m

g/L)

0

5

10

15

20

25

Cumulative ASR VolumeCalciumMagnesiumPotassium

Geochemistry of Stored Water

Minutes

0 20 40 60 80 100 120

THM

Con

cent

ratio

n (u

g/L)

0

10

20

30

40

50

60

70

80

ASR 2 (1800 gpm)ASR 4 (1800 gpm)ASR 5 (1100 gpm)

Time-Series THM Concentrations During Recovery

Findings• THM concentration appears to be related to storage

volume• Formation occurs rapidly during storage period• Rate of dissipation indicates that THMs are not

evenly distributed in stored water– Lack of observation in ASR 1 & 2 suggests a localized

phenomenom• Dissipation of THMs does not appear to be a

function of mixing/dilution (based on geochem data)

• TOC probably the limiting factor in formation• ∆ SUVA indicates potential source of additional

TOC/DOC

Next Steps• Continue to monitor during current storage period

to see if levels increase above existing concentrations– More frequent analysis during recovery operations

• Further analysis of SUVA during injection and recovery cycles at varying water elevations

• Examine HAA formation characteristics• Pursue dechlorination of injection water if

increasing concentration are not manageable