Field study of organic water quality changes during groundwater recharge in the Palo Alto Baylands

Water Res. Vol. 16. pp. 1025 to I035. 1982 Printed in Great Britain.All rights reserved

0043-1354 82,061025-11503.00,0 Copyright ~) 1982 Pergamon Press Ltd

FIELD STUDY OF ORGANIC WATER QUALITY CHANGES DURING GROUNDWATER RECHARGE IN THE PALO ALTO BAYLANDS PAUL V. RoBEg'rs, JOAN Scrm.Ex~ and GAgV D. HOPKINS Environmental Engineering and Science, Department of Civil Engineering. Stanford University, Stanford, CA 94305, U.S.A. (Received October 1981)

Al~tract--Water quality data arc presented from a field study in which reclaimed water is injected directly at a rate of 61 s- t into an aquifer in the Palo Alto Baylands on the margin of San Francisco Bay. Water quality changes are observed by analyzing samples from wells at distances of 10--40m and in differing directions from the injection point. Data on trace organic pollutants show evidence of retardation of movement in varying degrees, presumably caused by adsorptive interactions with the aquifer. Trihalomethane compounds show evidence of biodegradation in the aquifer. The concentration of total organic substance as measured by TOC and COD is decreased significantly by biodegradation, but total organic halogen appears unaffected by aquifer passage.

INTRODUCTION Groundwater recharge by direct injection of reclaimed municipal wastewater is the object of a field study in the Palo Alto Baylands begun in 1976 (Roberts et al., 1978). The aim of the program is: to rehabilitate a saline aquifer, to prevent intrusion of salt water from nearby San Francisco Bay, and to evaluate the long-term potential for augmenting the potable water supply. The facilities were constructed by the Santa Clara Valley Water District and are operated by the City of Palo Alto under a contract with the Water District. The facility consists of (1) a 0.09 m a s- t (2 million gallons d a y - t) water reclamation plant, and (2) a well field, approx. 1 x 3 kin, for injection and extraction of the reclaimed water. The reclamation facility is an advanced treatment plant in which secondary effluent is upgraded in a process sequence that includes lime treatment, air stripping, recarbonation, ozonation, illtration, granular activated carbon, and chlorination, The well field is conceived as a set of nine injection/ extraction well pairs with attendant monitoring wells. Details of the facilities design are given elsewhere (Roberts et aL, 1978; Jenks, 1974). Research has been conducted at the Stanford Water Quality Control Research Laboratory since 1976 with the goal of understanding water quality transformations that occur when reclaimed water moves through an aquifer following groundwater recharge, and to confirm expectations derived from theory and laboratory studies by comparing to field observations, In 1976-1977, a pilot study was conducted by injecting reclaimed water at a rate of 0.6-1.01s-' (10-15 gpm) into a well located at I2 (Fig. 1) (Roberts et al., 1978). An approach was adapted from chemical reactor theory for interpreting the water, quality observations at an observation well in terms of the re-

sponse to a step-change concentration stimulus at the injection point (Roberts et al., 1980). This approach was applied to characterize the transformation and transport of trace organic and inorganic pollutants during groundwater recharge. Thus, preliminary conclusions regarding the behavior of pollutants could be reached based on the I2 pilot study, despite the fact that the usable water quality data were limited to one observation well for most constituents because of the hydrogeologic properties of the aquifer near the injection point. The heterogeneity of the aquifer was so extreme that the injected water moved almost exclusively in a single direction. To overcome these limitations, the field study site was moved to well I 1, 300 m to the northwest (Fig. 1), where the aquifer was expected to be more homogeneous and the hydraulic transmissivity greater than at I2. The field study at I1 commenced in August 1978 and is planned for a duration of two to three years. The objectives are: 1. to test the methodology developed during the pilot study by verification with field data corresponding to longer times and distances and differing directions from the injection point; 2. to improve understanding of the behavior of pollutants, especially trace organic contaminants, in the groundwater environment; 3. to confirm evidence obtained in the pilot study that the movement of trace organic solutes is retarded significantly by sorptive processes; and 4. to evaluate the feasibility of predicting tbe arrival of pollutants at distant points based on short-term water quality studies at observation points near the injection point, in conjunction with an understanding of water movement.

1025

1026

PAUL V. ROBERTSet ell. ures at the sampling wells v,ere for the most part su~cient to guarantee artesian flow. Hence pumping was not necessdry to obtain water quality samples.

~ '~

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PALOALTO

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Fig. 1. Plan of groundwater recharge facility, This paper summarizes the results of the first year's field work at injection welt I1, with emphasis on the

behavior of trace organic contaminants,

EXPERIMENTAL APPROACH Description of field site The injection well designated as I1 is situated at the northwest end of the Palo Alto Baylands groundwater recharge field. The aquifer of interest is a permeable straturn of silty sand with some gravel, having a thickness of 1.5-3 m. and is found at a depth of 10-15 m below the ground. The aquifer is believed to be reasonably homogeneous and isotropic in the vicinity of well I1 (Charbeneau & Street, 1978), thus affording more nearly ideal hydraulic conditions than at the previous site, I2. The transmissivity of well I1 was determined in a step-drawdown test (Jenks, 1974) to be 110 m z day-I (9500 gallons day -t It-'), which implies a predicted injection rate of approx. 0.01 m s s-t (150gpm) at a well,-head pressure of 100 kPa clay (15 psig)' The i overlain aquifer l eexpected. is arelatively k by aimpermeable, ang imper" e meable layer and byunderlain interbedded layers of clay and silty sand. Bas~l on the welt logs, some upward is Six observation wells were available for water quality sampling in the vicinity of injection well I1, as shown in Fig. 2: P5, Pd, P7 a n d $22 at d i s t a ~ of 11, 20, 40 and 160 m along a straight line, and wells S23 and S24 at 16 m and 43 m in two other directions at angles of approx. 120" to the line II-$22. The observation wells P5, P6 and PT, from which most of the water quality samples in this work were drawn, are 2-cm (i],in.) threaded polyethylene tubes connected without adhesive, placed in 20-cm (8-in.) dinmater holes drilled by a hollow-stem auger without use of drilling fluidsl The polyethylene tubes were slotted to accommodate the full depth of the permeable stratum a n d placed accordingly. During injection operation, the press-

Analytical methods. Determination of specific organic compounds were made by gas ctaromatography. Highly volatile halogenated aliphatic compounds containing one and two carbon atoms were determined by a volatile organic analysis (VOA) procedure (Henderson et al.. 1976, Reinhard er al.. 1979: Trussell et al.. 1979)in which the organic solutes are enriched by pentane extraction in sealed hypovials prior to analysis. A 5-,ul aliquot of the extract was injected into a packed column gas chromatolzraph (lOg; squalane on chromosorb W/AW) equipped with a linearized 6SNi detector. (Reinhard et al., 1979). Chlorobenzene was concentrated by the closed-loop stripping (CLSA)method (Reinhard et al.. 1979; Grob & Zi.ircher. 1976) in preparation for ~as chromatography. In the CLSA method, organics are concentrated by circulating air through the sample and thence through an activated-carbon microtrap, which is subsequently extracted with carbon disulfide (Grob & ZiJrcher. 1976). The extract is injected splitless onto a glass capillary gas chromatograph equipped with programmed temperature control. Quantitation is achieved by comparison of the flame ionization detector signal with that of the CI-C~, internal standard (Reinhard et al., 1976). The detection limit for the compounds reported herein is approx. 0.1 p g l - t using the above methods. Total organic carbon was determined with a Dohrmann DC-52 organic carbon analyzer after digestion of 10,ml samples using the Oceanography International ampule system. Total organic halogen was measured on 50-ml samples as a sum of separate determinations of purgeable organic halogen (POX) by stripping and combustion of volatile organics, and of nonpurgeable organic halogen (NPOX) by two-step batchadsorption on powdered activated carbon, nitrate wash and combustion. The halogens chloride and bromide were on-line titrated by a microcoulometer. Composition of injected water and formation groundwater The approximate composition of the formation groundwater near well I1 prior to injection was substantially different from the average composition of reclaimed water during the study period, as shown in Table t. The more than 10-fold difference in chloride concentration permitted the use of chloride as a tracer in quantifying the movement of injected water. Concentrations of ammonia and of specific trace organic compounds such as trihalomethanes were below detection limits in the formation groundwater. DISTANCE FROM INJECTION WELL ~6o 40 ~o eo Io o ~6 4~ ,~ ¢,. ~ ~, " _

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Fig. 2. Aquifer section and layout of wells.

Organic water quality changes during groundwater recharge Table 1. Composition of injected water and formation groundwater Concentrations Injected water Formation average for groundwater at II July 1978prior to injection July 1979 COD, mg 1-x 0 : TOC, mg [-=C NHj-N, m g I - Z N Cl- mg l- * CI TTHM',/zg1-1

6 2 <1 4000 <0.2

10 3 16 290 13

• TTHM: total trihalomethanes, sum of concentrations measured by gas chromatography,

and more than ten times higher in the injected water. However. the concentrations of TOC and COD in the reclaimed water were not much higher than those in the formarion groundwater. The relatively low residual concentrations of COD and TOC in the reclaimed water cornpared to typical municipal effluents can be explained by the excellent performance of granular activated carbon in the reclamation plant. The average COD concentration of l0 m g l - t satisfies exactly the Regional Water Pollution Control Board's quality standard. The average concentration of total trihalomethanes comfortably satisfies the U.S. standard. 100pgl" ~.

Method of data interpretation The methodology used here for analyzing water quality changes occurring during groundwater recharge is based on a stimulus-response approach in which the aquifer is considered as a reactor vessel of unknown size, shape, and flow characteristics(Robertset al., 1980; Levenspiel & Bischoff, 1963). The approach is described elsewhere (Roberts et al., 1980: McCarty et al.. 1979). Retardation of solute movement is characterized by comparing the breakthrough for a solute of interest with that for a conservative tracer such as chloride. In this manner, useful quantities can be estimated, such as the relative rate of transport of a pollutant relative to water and the effective retention capacity of the aquifer with respect to the solute under the conditions of the field experiment,

Hydraulic performance During the first year of injection at well I1, covering the period from July 1978 to July 1979. a total of [25,000 m J (33 million gallons) of reclaimed water were introduced into the aquifer. The flow rate and injection pressure are shown in Fig. 3 as a function of the cumulative volume injected. The injection rate was controlled by manipulating the pressure at the well head. During the initial period of operation (<36,000 m~), the injection pressure was maintained in the range of approx. 70-I00 kPa ([0.-14 psi) and the resulting average flow rate was 6.31s-L (100 gallons min-I). A higher pressure, ll0-140kPa (16-20psi), was required to maintain the same flow rate when injection was recommenced following a 2-month shut-down in Oct.Nov. 1978. Thereafter, the rate declined to approx. 4 I s - I (63 gallons min -~) at 90kPa (13 psi). This decline in bydraulic capacity is a result of clogging of the injection well. and eventually will necessitate redevelopment of the well. RESULTS AND DISCUSSION

Breakthrough of injected water The arrival of injected water at the observation wells was estimated from measurements of chloride concentration in the injected water, in the formation groundwater, and at the observation wells after injection began. Breakthrough curves for several observation wells are shown in Fig. 4, based on smoothed values of the chloride concentration response. Comparison of the responses at wells P5, P6 and P7, reveals that a greater volume of water must be injected to achieve breakthrough as the distance from the injection well increases, as is expected. For example, a 5070 breakthrough of recharged water is achieved after injection of 3 0 0 m a at a distance of I I m (P5), 900 m a at 20 m (P6), and 6000 m a at 40 m (P7). If the aquifer properties were ideally uniform with respect to permeability and thickness, the injected volume required to reach breakthrough at an obserration well would be proportional to the square of the distance from the injection point. The data indi-

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INJECTION VOLUME

( IOOO M 3 )

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Fig. 4. Fractional breakthrough of injected water based on chloride measurements (smoothed responses).

Table 2. Estimated aquifer pore volume and average residence time Distance from Observation injection well, r well (m) P5 P6 P7 $23 $24

Aquifer pore volume. Vr (m~l

Average residence time* (h)

Vf,inr:

400 930 6300 340 3100

17 40 270 15 130

1.05 0.74 1.25 0.42 0.53

11 20 40 16 43

*Calculated at an average injection rate of 6.3 I s-

z.

cute slight deviations from that ideal situation. From the viewpoint of water quality investigations, the iraportant point is that substantial flow of recharged water was observed in all directions and complete breakthrough of recharged water was attained at all sampling points within 50m of the injection point. Data for well $22 at a distance of 160 m, not shown m

ocity is approximately half as great in the direction P 5 - P 6 - P 7 as in the directions $23 and $24. This deviation from ideal radial flow is small enough to preclude serious difficulties in interpreting water quality monitoring data.

Fig. 4. indicate poor hydraulic connection with the injection well. The residence time of water in the aquifer before reaching a given observation well was calculated as follows: the volume of pore water displaced was estimated by integrating the area above the chloride breakthrough curve: the pore volume was then divided by the average injection flow rate to estimate the average residence time. The results of these calcuiations are summarized in Table 2. The average injection rate was approx. 23 m 3 h-~ (100 gallons min-1) during the breakthrough period. The residence time for travel to $24 was less than to P7. although the two wells are approximately equidistant fi:om the injection well. Similarly. less time was required for recharged water to reach $23 than P5. although P5 is closer to the injection welt. This indicates that the aquifer's permeability may be less in the direction P5--P6-P7 than in the directions toward $23 or $24. Approximate residence time contours are shown in Fig. 5. The contours would be circles under ideal, isotropic flow conditions, but are obviously distended toward $23 and $24. Preliminary analysis indicates that the flow vel-

Transport of trace organics in the aquifer was studled by observing concentration changes at obser, vation wells following injection. This was possible for a number of halogenated organic compounds whose concentrations in the injected water exceeded I/~g I= t, compared to background concentrations less than 0.I/agl -~ in the formation groundwater. Results

Transport of trace oroanic contaminants

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Organic water quality changes during groundwater recharge

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Fig. 6. Breakthrough of chloroform at varying distances and directions.

were expressed as fractional breakthrough of the organic solutes, calculated as the ratio between the observed concentration at the observation well and the average concentration in the injected water. Results were smoothed by calculating three-point running averages to reduce the scatter caused by short-term concentration fluctuations. Fractional breakthrough data for chloroform at several observation wells are shown in Fig. 6. The sequential breakthrough at the wells is obviously in accordance with increasing hydraulic residence time, As shown in Table 2 and Fig. 4, the hydraulic residence times are ordered approximately as follows:

sidering the errors of estimation. The retardation of the compounds studied increases in the order CHCI3 < CHBr3 -~ CHCIBrz < CI3CCH3 < C~HsCI which is in approximate accord with the order of elution of these compounds in gas chromatography. The retardation is attributed to sorption of the cornpounds on aquifer material, and can be related to the hydrophobic nature of the organic solutes. Malcolm et a/. (1979) have shown that hydrophilic substances such as ethanol and carboxylic acids move through the groundwater environment with approximately the velocity of water movement, while the more hydrophobic halogenated compounds are retarded and

tr, ~ ts:., < ts.,~ < tp.

retained by aquifer material. Karickhoff et al. (1979)

which corresponds to the order of breakthrough Of chloroform (Fig. 6). Furthermore, it can be seen clearly by comparing the breakthrough curves for injected water (Fig. 4) with those for chloroform (Fig. 6) that chloroform appears later than the injected water; its movement is retarded compared to a conservative tracer such as chloride, Organic compounds differ in the degree to which their movement through the aquifer is retarded, as shown in Fig. 7. Breakthrough data for chloroform, bromoform and chlorobenzene at well P5 are compared to the injected water. Chloroform appears soon

have found excellent correlation between the sorption coefficient and the log octanol:water solubility as a measure of hydrophobic character. The extent of sorption is approximately proportional to the organic carbon content of the aquifer material (Karickhoffet al., 1979). Giger & Molnar (1978) have documented the persistence and slow movement in groundwater of tetrachloroethylene, a chlorinated hydrocarbon similar in structure to those studied here. The values of

after t h e i n j e c t e d water, while the appearance of bromoform is retarded to a greater extent and chloro-

benzene to a still greater degree. The estimates of retardation and retention in the shown in Table 3. The values of the retardation factors and the specific retention capacities were obtained by integrating the areas above the breakthrough curves (Roberts et al., 1980) for injected water and the organic solutes of interest. In calculating the specific retention capacity, values of the aquifer bulk density and porosity were assumed to be 2 x 105 kg m -3 and 0.25 (dimensionless). respectively,

The retardation factors estimated for chloroform at various observation wells show good agreement, con-

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VOLUME[)03m3 I

Fig. 7. Breakthrough of solutes at well PS.

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Table 3. Retardation factors and specific retention capacities for

Aquifer pore

Sample well

~olume* (m 3}

Compound

$23 P5 $24 P7 P5 P5 P5 P5

340 400 3400 6300 400 400 400 400

CHCI 3 CHC}3 CHCI~ CHCI3 CHBr3 CHCIBr, CI~CCH~ C,H,CI

A~erage injected concentration during breakthrough b-~gI- t)

Retardation factor

7.6 "4 6.7 6.0 6.6 2.6 3.3 2.0

3.5 3.8 2.5 3.0 6 6 12 33

trace

organics

Estimated specific retention capaci~? ag removed ( Ig aquifer material I 2.3

2.5 1.5 !.5 4.0 6 4.4 8.2

*Estimated from chloride data. the retardation factor for chloroform and chlorobenzene in the present study agree well with the values previously reported from the 12 pilot study: namely. 5 for chloroform and 36 for chlorobenzene,

volume subtended by the observation well: L. = V r x ('ii/i,:o) where

Attenuation o/concentrationfluctuations The concentrations of trace organics in the injected water fluctuate widely, owing largely to operational changes at the reclamation facility. These variations in input and the responses at observation wells are shown in Figs 8 and 9 for chloroform and chloro-

L, = Lag between extreme values in input and at the observation well. m 3 V r = aquifer pore volume subtended by the observation well. m 3 (t~/t,,0) = retardation factor, or ratio of residence times for the solute and injected water.

benzene. The concentration of chloroform at well P5 closely follows the input variation. Increased attenuation of the fluctuations is observed at wells $24 and P7, characterized by greater travel times and chloroform retentions than at P5. The attenuation of chlorobenzene at P5 is greater than for chloroform. The input variations in chlorobenzene concentration are scarcely reflected at P7. where chlorobenzene breakthrough has yet to be completed, The lag between maxima or minima in concentration input and the corresponding extremes at the

Lag values for chloroform and chlorobenzene are compared in Table 4. The agreement between the predieted and measured lags is encouraging. The attenuation of concentration fluctuations is expected to result in narrowing the probability distnbutions of concentrations measured at observation points, especially as the distance from the input increases. This behavior is illustrated in the form of log-normal distributions in Fig. I0 for chloroform. The decreasing slope of the regression lines in the order

observation wells can be correlated approximately with the values estimated from breakthrough data. The lag expected for a given compound at a given observation well is given by the product of the retardation factor for that compound times the aquifer pore

Injected water > P5 > P6 > $24 > P7

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9. Attenuation

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Organic water quality changes during groundwater recharge Table 4. Lags between injection input and observation well response

Compound

Well

CHCI3

P5 $24 P7 P5 P7

C~,HsCI

Range of lag following input variation (m s}

Predicted from breakthrough data

< 2000 5000-10.0(30 15.000-20.000 5(X)0-20.000 > 100,000

1000 8000 19,000 13.000 200.000

corresponds to increasing hydraulic residence time and subtended aquifer pore volume. A concentration peak encounters a greater aquifer volume en route to P7 than to P5. and hence is attenuated more strongly, The reduction in variability expressed as a spread factor is summarized in Table 5. This degree of attenuation is remarkable considering the relatively long period of the input fluctuations, i.e. 30.000-50,000 m 3 injected volume, or 2-4 months, Fluctuations of higher frequency would be attenuated more effectively by groundwater passage, Collective parameters Collective parameters including total organic carbon (TOC), chemical oxygen demand (COD), and total organic halogen (TOX) were measured as well as dissolved oxygen to obtain general indications as to the behavior of the bulk of organic substance injected into the aquifer, Data for TOC and COD as well as dissolved oxygen (DO) are shown in Table 6, comparing the values in the injected water during the period July 1978-Jan. 1979 with those at observation well P5, 11 m distant, There was a significant decrease in all three parameters during travel through the aquifer, all differences between I1 and P5 being significant at a confidence level of 99%. There is excellent agreement between the mean concentration changes for COD and DO, - 1.92 mg 1- 10z and - 1.94 mg I- 1 Oz. reto

~,-,;~d~o' *,;~',, . . . .

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Table 5. Spread factor for chloroform Spread factor

Sample point Injected water Observation wells: P5 P6 $24 P7

3.2 2.5 2.0 1.7 1.5

spectively, implying that the disappearance of dissolved oxygen is accounted for almost entirely by aerobic biodegradation of organic substrate. The ratio of the change in COD to the change in TOC is (1.92/0.43) = 4.5. This is rather high compared to t h e range expected for aerobic degradation to final products such as CO, and HzO, for which the values of the ratio (ACOD/ATOC) typically lie in the range between 2.5 and 3.5. The higher value of (ACOD/ ATOC) found here may indicate partial biochemical oxidation, leaving oxidized intermediate products. The responses of the TOC concentrations at wells P5, P6 and P7, following cyclic variation in the input concentration are illustrated in Fig. I1. The TOC concentration responds more rapidly and the amplitude of the variations is greater at the nearest observation well (P5) than at the farthest (P7). No conclusions can be reached regarding breakthrough or retardation of organic substances as measured by T O C or COD, because the background concentrations of the groundwater prior to injection were not greatly different from those of the injected water. In the previous pilot study at 12 (Roberts et a/., 1978), it was shown that the concentration of TOC responds as rapidly as a conservative tracer, hence indicating that the bulk of organics are transported at the velocity of water movement. Therefore, TOC concentration differences measured after complete breakthrough of injected water are indicative of removal of organics during aquifer passage. From the data in Fig. 11, it may be inferred that significant changes in

s

---sa4

Table 6. Concentrations of collective parameters in injection water and at nearest well

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Concentrations" Mean + SD

~o ao so eo so s¢ so ss % OF SAMPLES

WITH CONCENTRATIONLESSTHai GIVENI/Jd.LIE

Fig. 10. Cumulative probability distributions for chloroform at different distances from injection point.

P5

Decrease* II-P5

2.9+0.09 7.7+0.53 2.1 __+0.26

0.43+0.08 1.92 +0.67 1.94 + 0.35

11

TOC. m g l - t C 3.4+0.10 COD. mgI-IO., 9.6+0.67 DO. mg 1-' O, 4.08 + 0.35 "31 s e t s o f s i m u l t a n e o u s

samples

at

injection point

(I 1)

and n¢=rcst observation well P5 (11 m). Samples were taken during the period Aug.-Sop. 1978. Vnw = 1000-

35.000m s. +All differences significant at 9900 level of confidence.

1032

P~,UL V. ROBERTS et al.

%'., ~ ~.

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INJECTEO VOLUME[lOSm3]

Fig. 11. Variations in TOC concentrations at injection and observation wells.

TOC occur not just in the immediate vicinity of the injection point, but also during further travel beyond the first observation well P5 to the farther well P7. The results of the pertinent statistical comparison are summarized in Table 7. These data correspond to a time period subsequent to the period on which Table 6 is based. The injected concentration of DO was 2 m g l - t , compared to 4 mg 1-~ in the earlier period, The decreases in TOC from P5 to P7 as well as from II to P5 are significant at a confidence level of 99 percent. The COD concentration shows a significant decrease from P5 to P7, but not from I1 to P5. This seems an unreasonable result, and may be caused by errant COD measurements, There are significant decreases in dissolved oxygen both within and beyond P5; the greatest and most significant decrease is from I1 to P5. The mean dissolved oxygen concentration in the injected water over the period of this comparison was only 1,86 mg 1- t ; hence, the extent and rate of aerobic degradation was certainly limited by the availability of dissolved oxygen. Nonetheless, biodegradation

appears to have occurred even beyond P5, in a region where the dissolved oxygen concentration was substantially less than 1 m g l - L This observation is in disagreement with model predictions (McCarty et aL, 1979)that biodegradation should be completed to the extent possible within a few meters of travel from the injection point, and in view of the variability of the data should be interpreted cautiously.

Concentrationsof total organic halogen (TOX)in

samples from injection water and observation wells show similar behavior (Fig. 12). The data are limited to the period from March through July 1979. The pronounced drop in TOX concentration is attributed to the change from exhausted to fresh granular activated carbon (GAC) at the beginning of April, corresponding to a volume of 85,000 m 3. The virtually im, mediate response of the concentrations at both wells P5 and P7 following the change in input is obvious, There is no significant change or lag in concentration between the injection and observation points, Hence, the organic halogen must be associated for the most part with compounds that are both poorly degradable and poorly adsorbable. The mean TOX concentration is substantially higher than can be accounted for by gas chromatographic determination of specific compounds (Table 8). The molar concentration of TOX is approx. 5 times higher than the sum of moles of halogen found in specific compound analysis using the VOA and CLSA techniques, while the purgeable fraction alone of TOX was twice as high as the sum estimated from specific compounds. This indicates that the bulk ef the organically formed halogen may be incorporated into compounds sufficiently polar so that (a) they are difficult to analyze, and (b) are poorly retained by sorbing solids such as granular activated carbon and soil constituents. The existence of such polar, halogehated substances deserves special attention in future studies of water reclamation and groundwater recharge, as these substances seem to be capable of rapidly penetrating the defensive measures meant to retain hazardous pollutants, namely granular activated-carbon treatment and soil passage.

Table 7, Concentration differences for collective parameters at varying distances from injection point

TOC Concentration Concentration decrease

I1 P5 P7 I1 to P5 P5 to P7 11 to P7

2.8 _+ 0.32 2.1 + 0.18 1.3 + 0.34 0.71 + 0.17~ 0.69 + 0.14§ 1.4 + 0.27~

Concentrations* Mean __. SD, mg 1COD 11.2 __+1.0 11.0 _ 1.1 7.7 + 1.9 0.12 + 1.9 3.4 + 1.25~ 3.5 + 1.1~

DO 1,86 + 0.56 + 0.35 + 1.25 + 0.2 + 1.5 +

0.42 0.14 0.05 0.27.~ 0.13"t" 0.43~

*Sixteen sets of simultaneous samples at injection point (I1) and observation wells P5 111 m) and P7 (4Ore); samples were taken during the period Dec. 197g--July 1979, Viw = 40,0(0-125(100 m 3. I"~§Levels of confidence that the m~n difference is significantly different from zero. ~'P/> 0.~0: ,+P t> 0.95; ~P >t 0.99.

Organic water quality changes during groundwater recharge a~o

,

,

'~'

•

1033

Table 8. Halogenated organics measured

o x-)

in injected water

I~ P 7

Concentration Organic halogen: Total Purgeable ~)ool-

°

1.67 ( I00'!,,1 0.33 (20%)

Sum of halogen in specific compounds

0.16 (lO°,)

I-

°,

~so x )=

rate constant of 0.03 day-* until a plateau of

=

075/agl -~ is reached after 150 days. This plateau

0

may represent the lower limit attainable by degradaDF,,s~ r~ ,rR),O,.)TY t~IZ.~TII)N

¢o d~ 9b do ,b ~

tion

of c h l o r o f o r m

under

the

conditions

in

the

aquifer. The concentrations of the other three trihaloINdECTED VOLUME (lOSm ,) methanes subsided below the detection limit of 0.1 pal -1 within 150 days. Fig. 12. Total organic halogen (TOX). Possible explanations include (1) biodegradation. (2) sorption, or (3) dilution by advective flow of groundwater containing lower concentrations of the Field eridence of hiodegradation of halogen°ted or- chlorinated organics. Biodegradation is the most ganics plausible explanation for the decline in concentrations The accumulation resulting from adsorption of oftrihalomethanes. The order of magnitude difference hazardous pollutants such as halogenated hydro- in the rates of concentration decrease for the trihalocarbons in an aquifer raises questions concerning methanes compared to the two-carbon compounds their ultimate fate. The extended residence times of provides a strong argument against the dilution or organic pollutants adsorbed in the aquifer afford adsorption explanations. Even if the slow decrease in opportunities for biodegradation of organic c o r n - concentrations of two-carbon compounds were pounds even if they are only slowly degradable, caused by dilution, the higher rate of decrease of triHence. it is meaningful to seek evidence of biodegra- halomethanes could not be accounted for by the same dation that may occur slowly within a time frame of explanation. Moreover. it is known from isotope years, tracer measurements that the rate of groundwater Such an experiment has been conducted over a year movement is very slow in the immediate vicinity. at the pilot study site I2 subsequent to terminating Furthermore, sorption capacity of the aquifer for all injection there in June 1978. At the time that injection of these compounds was known to be saturated ceased at 12, the aquifer at the nearby observation before the degradation experiment began, as shown well* was saturated with a number of chlorinated by- by their concentrations at the observation well being drocarbons, as evidenced by their complete break- not significantly different from the concentrations through. Samples of the groundwater at the obser- injected (Roberts et al.. 1980). vation well were taken periodically and analyzed for volatile chlorinated organic compounds using the VOA method described above. Data are shown for several halogenated hydrocarbons in Figure 13. ,oo~ Within 50 days the concentrations of chloroform and sol ~ c,,c. c.c, other trihalomethanes were found to have decreased I . c,,cc., significantly. The concentrations of the trihalometh~ c,,c. :% 2¢ " ~ * CHCI~ anes decreased at a rate of 0.03 day-i, nearly ten ~ ,c x • c.c,,s, ., • CHClor z times as rapidly as the rate of concentration decrease .,.. .,a-,.....,,'x~~ &_, .)ooo3E**,, ] • c..,, for the group of compounds containing two carbon g ~ * ~ /-~'**"''"~L~ ---, atoms, represented by trichloroethylene, tetrachloro- ~ g~ ~----..~. ethylene, and l.l.l-trichloroethane. ~" " ~ ' g ~ The presumed degradation of trihalomethanes is ,~ • . ~ . best illustrated by the behavior of chloroform in Fig. ~ o 13. The concentration of chloroform remains constant ~3111 1 at the initial value of 20 pg I- t over an initial period of 20--30 days. followed by a first-order decline with a o, . . . . . . . . . . . . . . . . . . . . . . . ,~o

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

~0

t00

"p

T,.C

• Observation well P4. 8 m distant from injection well 12.

200

CoAYs3

250

~

Fig. 13. Field evidence of long-term degradation.

350

[034

P~EL V. ROBERTS ~<" ,aI'

CONCLUSIONS

Nonetheless. some organic pollutants do penetrate through both water reclamation and groundwater recharge, mm, ing with approximately the same ',elocit.~ as the recharged water toward points of eventual extraction for reuse. This is illustrated b? the rapid transport of halogenated organic compounds measured as TOX, More detailed characterization of these rapidly migrating substances must be achieved before their public health significance can be evaluated. A methodology for interpreting water-quality data from a large-scale groundwater recharge operation has been tested by analyzing time series of samples taken at different directions and distances from the point of injection. Information can be obtained over a period of several months to a year that is useful in predicting long-term water-quality changes. Continual observations over longer distances and times are needed to confirm the reliability of model predictions. The methodology demonstrated and the results presented herein are believed to be significant for situations other than groundwater recharge by direct injection. The transport rates and fates of organic contaminants are of importance in evaluating the health risks of groundwater pollution regardless of its source. Organic comtaminants may enter groundwater other than by direct injection: for example, as a result of spills, sanitary landfills, agricultural practices, land treatment of wastewater, or by surfacespreading recharge. Upon reaching the groundwater zone, the pollutants so introduced are likely to be influenced by the processes found in this work to be important, namely sorption and biodegradation.

The concentrations of organic constituents are affected by processes such as adsorption and biodegradation during groundwater recharge with reclaimed water, even where the recharged water has received advanced treatment including granular activated carbon. The influence of adsorption is observed as retardation of the transport of trace organic cornpounds, which are retained to varying degrees in the aquifer. The effects of biodegradation are evidenced by decreasing concentrations of TOC. COD, and dissolved oxygen as the water spreads out from the injection point. Generally, the concentrations of TOC and COD respond rapidly to fluctuations in input concentration, indicating that the bulk of organic constituents in the reclaimed water do not engage in sorptive interactions with the aquifer material. Surprisingly, halogenated organic compounds measured by TOX show no evidence of retardation or retention during aquifer passage, The accumulation of trace organic compounds in the aquifer by sorption has several implications. The retardation of movement means that strongly sorbing pollutants will be delayed many years before reaching the point of water extraction for reuse, if the hydraulic residence time between recharge and extraction is on the order of months or years. Furthermore. the long residence times in the aquifer that result from sorptive retention provide opportunities for biodegradation to occur to a significant extent, even if degradation is very slow. There is presumptive evidence of trihalomethane degradation in recharged groundwater under anoxic conditions. Moreover. variations in concentration input are strongly attenuated by sorptive in- Acknowledgements This work was supported by the teractions with aquifer material, as well as by the less Robert S. Kerr Environmental Research Laboratory, U.S. selective process of hydrodynamic dispersion. Hence, Environmental Protection Agency. Additional funding was short-term fluctuations in the concentrations of provided by the State Water Resources Control Board and strongly sorbing contaminants are of little concern for Department of Water Resources, State of California. The Santa Clara Valley Water District generously made availgroundwater recharge, and occasional extreme values able its water reclamation and groundwater recharge faare not cause for alarm. Understanding of the above cility.The authors are indebted to Dr. Martin Jekel for the factors is important in designing a rational, cost-effec- TOX data. R. Scott Summers assisted in data reduction. tive monitoring program for trace organic pollutants. This paper was presented at the Symposium on Wastewater Reuse for groundwater Recharge. Pomona. Ca/iforThe sequential breakthrough of trace organic con- nia, September 1979. taminants is believed to be caused by differing strengths of sorption on the aquifer material. Retardation factors estimated for this field site range from a REFERENCES value of 3 for chloroform to 35 for chlorobenzene. Charbeneau R. J. & Street R L. 119781 Finite-element The rank of retardation is in approximate agreement modeling of groundwater injection-extraction systems. with the order of elution in gas chromatography, and Technical Report No. 231. Department of Civil Engmis believed to be governed largely by sorption on oreering,Stanford University. CA. ganic soil constituents as a result of the hydrophobic Giger W. & Molnar-Kubica E. 11978t Tetrachloroethylene in contaminated ground and drinking waters. Bull, encir. character of the organic molecules, contain. Toxic. 19, 495. The organic pollutants remaining after water recla- Grob K. & Zilreher F. [1976) Stripping of organic submation treatment pose a potential health threat if the stancesfrom water. J. Chromatogr. 117. 285. reclaimed water is to be directed to potable reuse. Henderson J. E., Peyton G. R. & Glaze W. H t1976) A Groundwater recharge by direct injection affords proconvenient liquid-liquid extraction method for the determination of halomethanes in water at the parts per biltection against this risk by affording opportunities for lion level. Identification and Analysis of Oryanic Poll,adsorption and degradation and by smoothing contants in Water (Edited by Keith L. Ft.). Ann Arbor centration fluctuations. Science. Ann Arbor. Mi.

Organic water quality changes during groundwater recharge Jenks J. J. (19741 A program for water reclamation and groundwater recharge. Predesign Report to the Santa Clara Valley Water District. Karickhoff S. W.. Brown D. S. & Scott T. A. (1979) Sorption of hydrophobic pollutants on natural sediments, Water Res. 13, 241. Levenspiel O. & Bischoff K. B. (19631 Patterns of flow in chemical process vessels. Adrances in Chemical Engineering {Edited by Drew T. B.. Hoopcs J. W. Jr & Vermuelen T.). Vol. 4. pp. 95-198. Academic Press, New York. Malcolm R. M.. Thurman E. M.. Aiken G. R. and Avery P. A. (1979) Hydrophilic organic solutes as tracers in groundwater recharge studies. 177th National Meeting of the Americalz Chemical Society. Honolulu. McCarty P. L.. Rittnann B. E. & Reinhard M. (19791 Processes affecting the movement and fate of trace organics

1035

in the subsurface environment, gymlxs.~ium on Wa,~te~'ater ReuseJ'or Groundwater Recharge Pomona. CA. Reinhard M., Everhart E. T.. Schreiner J. E. and Graydon J.W. (19791 Specific compound analysis by gas chromatography and mass spectrometry. NATO/CCM$ ConiC,rence on Adsorption Techniques. Washington. DC. Roberts P. V.. McCarty P. L. & Roman W. M. 11978) Direct injection of reclaimed water into an aquifier, d. envir. Engn# Div. Am. Soc. ciu. Engrs 104, 933. Roberts P. V., McCarty P. L.. Reinhard M. & Schreiner J. E. (1980} Organic contaminant behavior during groundwater recharge. J. War. Pollut. Control. Fed. 52, t61. Trussel A. R., Umphres M. D.. Leong L. Y. C. & Trussel R. P. (19791 Precise analysis of trihalomethanes. J. Am. War. Wks Ass. 71,385.