Extractable Organic Matter in Urban Stormwater Runoff. 1. Transport Dynamics and Mass Emission Rates? Robert P. Eganhouse" and Isaac R. Kaplan Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90024
Los Angeles River waters were collected during the progress of a storm event in 1978 and analyzed for solvent-extractable organic substances. The extracts from both filtered and unfiltered samples were fractionated into five compound groups of differing polarity. No statistical relations were found between gross storm parameters and the organics carried by the runoff waters; however, the various fractions showed high intercorrelations. The majority of the extractables are associated with particulate matter; their partitioning between particulate and dissolved phases is controlled primarily by solubility. Mass emission rate estimates indicate that the Los Angeles River contributes nearly 1%of the annual world petroleum hydrocarbon input to the ocean via urban runoff.
Introduction Present-day estimates for the input of petroleum residues to the ocean via surface runoff are on the order of 1.9 x IO6 metric tons yr-l ( I ) . Calculations of this sort are fraught with serious drawbacks stemming primarily from a lack of reliable data. Clearly, it is impractical to routinely monitor urban runoff and nonurban river discharges on a regional or even local scale. However, the accuracy of our estimates could be greatly improved if we were able to delineate the transport dynamics involved. Furthermore, when considering surface runoff, one should make a distinction between the chronic discharge typical of continuously flowing rivers and the episodic behavior of seasonal rivers and streams. In the present study, the Los Angeles River, a seasonally active urban drainage system, was selected for two reasons. First, it is the single most important river in southern California, accounting for -28% of the total yearly flow ( 2 ) .Second, it drains a large portion of the highly urbanized Los Angeles basin, as well as surrounding undeveloped areas. Thus, both anthropogenic and natural terrigenous materials are likely to be significant constituents of any transported organic matter, The river empties into a coastal environment presently receiving input of organic materials from a multitude of sources ( 3 ) .Therefore, detailed molecular and isotopic ( 4 )characterization of river-borne organics may aid the ultimate differentiation of these sources to coastal marine sediments. In undertaking this project, we had three objectives in mind: (1)to study the transport of extractable organics in relation to dynamic features of the storm itself, (2) to investigate those factors controlling the distribution of organic molecular types between particulate and dissolved phases, and (3)to formulate mass-emission estimates for various organic compound types, including petroleum hydrocarbons, issuing into southern California's coastal waters via surface runoff. Details of the molecular ( 5 ) and elemental/isotopic (6) characteristics of these stormwaters will be presented subsequently. + Publication No. 2045, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024.
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Environmental Science & Technology
Study Area The modern-day Los Angeles River largely comprises man-made channels stretching from the San Gabriel Mountains to San Pedro Harbor (Figure 1).Under storm conditions, the flow is regulated by a number of reservoirs within the drainage basin. Mountainous, hilly, and coastal plain topographies characterize the more than 2100-km2basin, and a major part of metropolitan Los Angeles is drained by the system. Because of the high coverage of city surfaces by impermeable materials, infiltration is low, and storms can generate large runoff flows ( 7 ) . The rain pattern for the storm studied here (Figure 1) indicates that very little precipitation occurred in the mountain areas where the population density is low. Instead, the highly industrialized east central and harbor (south central) sections of the city experienced the greatest rainfall accumulation. Although small by comparison with other storms that occurred during 1978-1979, this storm event was the second of the rainy season. Consequently, flushing of organic residues that accumulate during the dry season may have been particularly effective. Experimental Section Sampling. On November 21, 1 9 7 8 , l l duplicate samples of runoff were collected a t various times during a storm near the mouth of the Los Angeles River (Figure 1).Water samples were obtained from the middle of the channel by submerging a vaned 4-L steel collection device just below the stormwater surface. Upon retrieval, the sampler was thoroughly shaken to induce homogenization, and equal volumes were poured alternately into two glass containers. This procedure was repeated until exactly 3 L had been added to each bottle. After returning to the laboratory, we added 200 mL of hexane to one of the two bottles collected at each sampling time. This was done to prevent evaporative losses and to initiate extraction. A 100-350-mL subsample was removed from each of the other (second) bottles for total suspended solids (TSS) determination ( 8 ) ,and then 2 L more were filtered through solventcleaned, precombusted glass-fiber filters (Whatman GF/A). The filtrates were transferred to clean glass containers and preserved with 200 mL of hexane. Thus, filtered (2 L) and unfiltered (3 L) samples corresponding to each of 11sampling intervals were obtained. The contents of the bottles were spiked with HgClz to terminate biological activity and stored at 10 "C until extractions could be performed. The remaining sample (-900-650 mL) was also poisoned with HgCl2 and allowed to sit for 48 h, after which the supernatant was removed and the settled solids were stored frozen for elemental and isotopic analysis. Analyses. Liquidhiquid extractions were performed successively on all samples with the 200-mL hexane preservative followed by three portions of CHC13 (300 mL/3 L samples). The combined extract for each sample was concentrated to a small volume by rotary evaporation under reduced pressure at 30-35 "C, and the concentrate was dried with anhydrous NaZS04. The extract was then passed over activated copper 0013-936X/81/0915-0310$01.25/0 @ 1981 American
Chemical Society
LOS ANGELES
FLOOD CONTROL DISTRICT
+ Rantdl statim 415
-p
Runoff sampling station Rainfall cantcur mIervol*O 21nche
I,
TIME
Figure 2. Flow, suspended solids, and rainfall variations during the storm of November 21, 1978. 4 ’
Figure 1. Map of the Los Angeles River drainage basin showing rainfall accumulation contours and runoff sampling station for the storm of November 21, 1978.
to remove elemental sulfur, reduced to a known volume, and analyzed gravimetrically on a Mettler ME22 electrobalance for total extractable organics (TEO). A portion of each extract was removed for methylation of extractable fatty acids by a modification of the BF3-MeOH technique (9).We were concerned that solvent-soluble triglycerides and nonmethyl esters of fatty acids might not be quantitatively transesterified by this procedure (10). Therefore, we compared the above method with a saponification/esterification procedure ( I I ) using three of the samples. The fatty acid methyl ester (FAME) yields and molecular distributions were identical within experimental error for the two techniques. Separation of the total extracts into five fractions was achieved by thin-layer chromatography using plates coated with 0.25 mm of silica gel G and developed in CH2C12. All plates were precleaned by running once in MeOH and twice in CH2C12 and removing the top 1 cm of silica gel after each elution. Five fractions were isolated: (1) total hydrocarbons (THC), R f = 0.77; (2) fattyacids (methylesters) (FA), R f = 0.63; (3) ketones (KET), Rf = 0.50; (4) polars (PLR), 0.48 2 R f 1 0.02; (5) nonelutable polar compounds (NEP), R f = 0. The PLR fraction contains hydroxy FAMES, dicarboxylic acids, and a variety of polyfunctional moieties in addition to fatty alcohols and sterols (5).Total hydrocarbon, FA, KET, and PLR bands were eluted from the silica gel and analyzed for their content gravimetrically; the NEP fraction was estimated by mass balance. Three THC samples were further separated by thin-layer chromatography into aliphatic and aromatic fractions by pentane elution on silica gel G: aliphatics, Rf = 0.95; aromatics, R f I0.92. Only gravimetric results are presented here; gas-chromatographic and gaschromatographic/mass-spectrometric identification of the fractions will appear elsewhere ( 5 ) .
Results The general flow characteristics and suspended solids distribution for the storm are presented in Figure 2. Streamflow data were determined by water stage recording stations located near the sample collection site (Figure 1). Figure 2 also contains cumulative rainfall data for station 415 on this day.
The apparent lag between rainfall and streamflow is related to the discharge characteristics of the entire river system whose drainage basin is more than 65 km long in linear north/south distance. As indicated in Figure 2, stormwaters were sampled from the beginning of the storm until just after peak flow had occurred. During this time, suspended solids levels generally increased. The data for total extractable organics and the five fractions are summarized in Table I. Unfiltered samples showed a high variability for all measured parameters. In contrast, relatively constant levels were found for THC, FA, and KET fractions in filtered samples. Values for the “particulate” concentrations of various fractions were computed by subtracting data for filtered samples from that of the corresponding unfiltered samples. Mean values for the calculated “particulate” concentration of THC and its “dissolved” (i.e., filtered) counterpart were 10.5 f 6.4 and 0.39 f 0.05 mg/L, respectively. MacKenzie and Hunter (12)found values in a Philadelphia storm sewer to be 2.7-5.1 mg/L (particulate) and 0.16-0.34 mg/L (dissolved); Zurcher et al. (13) noted similar levels in motorway runoff in Switzerland. Figure 3 illustrates the data plotted against time for both filtered and unfiltered samples. Several features are readily apparent. First, in the unfiltered samples, all parameters show an increase in value a t 1100 hours similar to the behavior of suspended solids (cf. Figure 2). Secondly, there is a marked increase from 1300 to 1500 hours. This change seems to correspond to the behavior of the flow during the same time. Statistical analysis, however, failed to yield a strong correlation between flow or suspended solids and any of the organic parameters (Table 11).The apparent similarities in the concentration trends for TEO and the various subfractions are borne out by the high correlation coefficients obtained by linear regression analysis (Table 11).Thirdly, comparison of the temporal variations for filtered and unfiltered samples of the same fraction indicates that only in the case of TEO and the PLR fraction is there a degree of resemblance ( r = 0.87 and r = 0.80, respectively). The filtered THC data show an extremely small variation (coefficient of variation = 12.4%) considering the wide fluctuations evidenced in the unfiltered samples. In agreement with the findings of others (12-15), most of the extractables and, in particular, the hydrocarbons are associated with the filterable particulate matter (Table 111).In the case of the hydrocarbons, the percentage in the “dissolved” phase relative to the total burden is consistently below 15% Volume 15, Number 3, March 1981 311
Table 1. Summary of Gravimetric Data for Total Extractable Organics and Subfractions Recovered from Storm Runoff Samples a sampllng the, h
total extractable organlcs U F
1000
2.55
1100
14.2 6.25 9.13
1200 1300 1400 1450 1500 1550 1600 1700 1800
total hydrocarbons U F
1.55 2.52
0.99 6.22
1.85 2.27
2.54 3.96
18.1 26.0 30.8
2.00 4.20 4.22
10.5 19.0 18.2
33.3 25.7
4.80 3.53 3.02
19.5 10.4 9.21
2.64
15.9
17.7 27.7
lalty acids
0.194 1.12 0.449
0.378 0.360 0.410 0.490 0.366 0.388
0.912 1.41 1.96 4.27
0.405 0.357
2.75 2.37
0.432 0.302
1.15 1.72
All data in mg/L. U = Unfiltered sample. F = Filtered sample.
36
I
I
I
I
I
I
Untiltered
z4
-
:
........ Total rxtractable organlca (TEO)
+Toial hydrocarbonr(THC)
---
Fatty acid#(FA)
- Ketones(KET)
f!
0.124 0.095 0.102
0.560 0.163 0.375 0.700 1.12 1.49
0.062 0.045 0.059 0.068 0.053 0.143
1.67 1.58 0.678 0.858
0.136 0.042 0.042
F
0.967 2.10 0.958 1.43 2.22 4.42
0.154 0.564 0.469 0.528 0.930 0.978 1.56 1.45 1.15 0.618 0.893
4.69 5.40 4.43 3.80 5.61
0.049
0.266 4.20 2.14 2.45 3.27 C
2.15 3.98 6.92 2.86 3.61
0.846 1.39 0.880 1.17 0.400 2.59 2.00 2.72 1.85 1.80 1.30
Table II. Correlation Coefficients from Linear Regression of Gross Storm Parameters, Total Extractable Organics, and Organic Subfractions (Unfiltered Samples)
i.,
"..TEO
+ hiare (PLR)
... u
I
.
