Dredging Contaminated Upper Mississippi River Bottom

Vol. 39, No. 4

APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Apr. 1980, p. 782-789 0099-2240/80/04-0782/08$02.00/0

Bacteriological Water Quality Effects of Hydraulically Dredging Contaminated Upper Mississippi River Bottom Sedimentt D. JAY GRIMES

River Studies Center, University of Wisconsin-LaCrosse, LaCrosse, Wisconsin 54601

Bacteriological effects of hydraulically dredging polluted bottom sediment in the navigation channel of the Upper Mississippi River (river mile 827.5 [about 1,332 km] to 828.1 [about 1,333 km]) were investigated. Bottom sediment in the dredging site contained high total coliform densities (about 6,800 most-probablenumber total coliform index per g [dry weight] and 3,800 membrane filter total coliforms per g [dry weight]), and fecal coliforms comprised an average 32% of each total coliform count. Total coliform and fecal coliform densities in water samples taken immediately below the dredge discharge pipe were each approximately four times corresponding upstream vaules; fecal streptococcus densities were approximately 50 times corresponding upstream values. Correlation analysis indicated that mean turbidity values downstream to the dredging operation were directly and significantly (r > 0.94) related to corresponding total coliform, fecal coliform, and fecal streptococcus densities. Salmonellae and shigellae were not recovered from either upstream or downstream water samples. Turbidity and indicator bacteria levels had returned to predredge values within less than 2 km below the dredge spoil discharge area at the prevailing current velocity (about 0.15 mWs). The microbiological effects of hydraulic dredging were first investigated and reported by my laboratory in 1974 (21). That report concerned a maintenance dredging operation conducted in navigation pool no. 8 of the Upper Mississippi River. Fecal coliform (FC) concentrations in water samples collected downstream (about 0 to 1 km) from the dredging were significantly greater than in upstream samples. Increased counts in downstream samples were attributed to the disturbance and relocation of bottom sediment by dredging with concomitant release of sediment-bound FC to the overlying water column. Enteric pathogens and indicators other than FC are continually entering the Upper Mississippi River; once present, they are subject to sedimentation. This is given credence by noting that one of two swimming-associated outbreaks of shigellosis that occurred in the United States between 1961 and 1975 (9) was attributed to swimming in the Dubuque, Iowa, stretch of the Upper Mississippi River (6, 34). The outbreak (45 cases) was caused by Shigella sonnei and was epidemiologically linked to swimming in that heavily contaminated reach of the river (mean FC count was 17,500/100 ml). It is reasonable to assume that sediment beneath this

water contained high numbers offecal indicators and enteric pathogens (24, 37), and such organisms have been shown to persist in sediment for significant periods of time (19, 20, 24, 35, 37; P. LaLiberte and D. J. Grimes, manuscript in

t Contribution no. 11, River Studies Center, University of Wisconsin-LaCrosse.

Sample site. Dredging was conducted in the Grey Cloud Slough area of navigation pool no. 2 of the

preparation). Presumably, if dredging promotes resuspension of sediment bound FC, it should resuspend all sedimented bacteria, including other fecal indicators and enteric pathogens. However, this supposition has not been documented in published literature. For this reason, a study was undertaken in the summer of 1976 to determine the bacteriological effects of hydraulically dredging bottom sediment known to be heavily contaminated with metropolitan sewage effluent. (The data in this paper were presented in part at the 78th Annual Meeting of the American Society for Microbiology, Las Vegas, Nev., May 1978. The data are also part of a Great River Environmental Action Team report [GREAT I report] released March 1978 entitled "A Pilot Study on Effects of Hydraulic Dredging and Disposal on Water Quality of the Upper Mississippi River (July 1976)," Federal Building, Fort Snelling, Twin Cities, Minn.)

MATERIALS

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AND METHODS

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WATER QUALITY AFTER HYDRAULIC DREDGING

Upper Mississippi River. The dredging was performed by the U.S. Army Corps of Engineers' hydraulic dredge William A. Thompson as part of its channel maintenance program. Dredged material (about 29,400 I3) came from bottom sediment in that stretch of river extending from river mile 828.1 (about 1,333 km) downstream to mile 827.5 (about 1,332 km) (Fig. 1). Chronologically, the dredge cut on 7 July 1976 extended from mile 828.1 downstream to mile 827.9 and the dredge cut on 8 July 1976 extended from 827.8 to 827.5 (Fig. 1). Dredged material disposal sites on 7 and 8 July 1976 were at miles 827.8 and 827.6, respectively (Fig. 1). Background data collected by the U. S. Geological Survey showed that current velocities averaged 0.15 m/s, mean discharge was 72.6 m3/s, water temperature was 25.9°C, biological oxygen demand was 4.1 mg/l, pH was 7.9, suspended solids were 12 mg/l, and dissolved oxygen was 2.4 mg/l in the study area (GREAT I report). The average channel depth in the study area was 4.6 m (range, 2.7 to 7.3 m). Water in the area was used for commercial and recreational navigation, swimming, water skiing, and fishing. Sampling techniques. Water samples were obtained from the overflow line of a nephelometric turbidimeter (Hach surface scatter model 2426, Hach Chemical Co., Ames, Iowa) aboard the R/V Izaak Walton. The turbidimeter was fed by a 2-inch (about 5-cm) intake, 1-hp (746-W) electrical centrifugal pump (Red Jacket pump, Davenport, Iowa) and was connected to a strip chart recorder (model LllOlS, Esterline Angus, Indianapolis, Ind.). Samples were collected in sterile 1-gallon (about 3.8-liter) polypropylene milk bottles and were immediately refrigerated (4°C) until they could be processed. Processing was done aboard the R/V Izaak Walton and always occurred within 4 h of sampling. Sediment samples were obtained on 6 July 1976 (before dredging) with the use of a Petite Ponar grab dredge (Wildlife Supply Co., Saginaw, Mich.). The samples were obtained from four different sites within each of the two proposed dredge-cut areas (Fig. 1). Three Ponar grab samples were collected at each site and were pooled and thoroughly mixed in a sterile aluminum foil baking pan. All eight sediment samples were immediately refrigerated (4°C) and were processed within 2 h onboard the R/V Izaak Walton. Indicator bacteria. All water samples were examined for the presence of membrane filter (MF) total coliforms (TC), FC, and fecal streptococci (FS) by filtering appropriate decimal volumes (0.1, 1.0, and 10.0 ml) through type HC membranes (HCWG 047 Si, Millipore Corp., Bedford, Mass.). TC were detected with mEndo agar MF (Difco Laboratories, Detroit, Mich.), FC were detected with mFC agar (Difco), and FS were detected with KF-streptococcus agar (Difco); mFC agar plates were immediately incubated in a 44.5°C water bath (Coliform Incubator/Bath, GCA/ PRECISION Scientific, Chicago, Ill.). Standard materials and methods were used (1), and duplicate analyses were performed on each sample. Sediment samples were also examined for TC, FC, and FS. Standard most-probable-number (MPN) procedures (1) were run on the eight samples and were paralleled with MF tests of sediment elutriates. The sediment elutriates were obtained by a modification of

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the elutriate test as described by the U.S. Environmental Protection Agency in 1975 (12). The modification consisted of adding 20 g of each sediment sample to 80 ml of sterile phosphate buffer (1) contained in a sterile 250-ml Erlenmeyer flask. This slurry was vigorously mixed on a platform shaker for 30 min and then allowed to settle for 1 h. The resultant liquid phase, or elutriate, was then subjected to MF analyses for indicators as already described. Recently, the elutriate test (12) has been refined (13), and the liquid phase used for MF analyses would be more correctly referred to as the "suspended particulate phase" (13). Enteric bacteria. Water and sediment samples were examined for salmonellae and shigellae by broth enrichment of filtrates collected on absorbant pad prefilters (AP10 047 Si, Millipore Corp.) and type HC membrane filters. Filtrates were collected by filtering 100-ml volumes of water and 10-ml volumes of sediment elutriates. Salmonella enrichment was done by placing one-half of the pad-membrane combination into tetrathionate broth (modified by the addition of 10 mg of brilliant green per liter of broth) and incubating at 41°C. Each broth was streaked onto bismuth sulfite agar (Difco) and XLD agar (Difco) at both 24 and 48 h. The other half of each pad-membrane pair was placed into GN broth (Difco) and incubated at 35°C for shigella enrichment. GN broths were streaked onto XLD agar at 24 h. Typical colonies were transferred to triple sugar iron agar slants (Difco), and all alkaline/acid cultures were checked for urease activity in urea agar (Difco). Urease-negative cultures were streaked onto MacConkey agar (Difco) to ensure purity; typical, isolated indophenol oxidase-negative colonies were transferred to tryptic soy agar slants (Difco). These slant cultures were then Gram stained and characterized by using SIM medium, Simmons citrate agar, MRVP broth, phenylalanine malonate broth, gelatin, and lysine decarboxylase medium (all from Difco). Cultures giving reactions consistent with Salmonella and Shigella (11) and three positive control Salmonella were then grown on veal infusion agar (Difco) and serogrouped with salmonella 0 antisera (Difco).

RESULTS Indicator bacteria. All sediment samples except E-2 contained large amounts of organic silty clay. Sample E-2 was predominantly medium sand. The sediment samples contained high densities of the two coliform groups, but relatively low numbers of FS. Table 1 lists MPN indices and MF counts for each of the three indicators in sediment, as well as FC/FS ratios. FC comprised an average 32% of each TC count (Table 1), and FC/FS ratios were strongly suggestive of human fecal pollution (18). Elutriate counts averaged 55% of MPN indices. Indicator bacteria, turbidity, and FC/FS values for water samples are listed in Table 2. The position of each sample relative to the dredge effluent pipe is shown, and each position has been corrected for the location of the two

APPL. ENVIRON. MICROBIOL.

GRIMES

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TABLE 1. Number of indicator bacteria isolated No. of bacteria/g (dry wt) of sedimenta

Sample no.

FS

FC

TC

FC/FSSd

MF

MPN

MF

~TO

CFU/lOOml

FS

MPN MFc MPNb 415.0 2 0.94) between turbidity values averaged according to sample po-

sition and each of the indicator bacteria mean densities averaged according to sample position (Table 2); this same relationship was observed under other circumstances by other investiga-

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GRIMES

tors. Saylor et al. suggested that TC, FC, and FS were associated with suspended sediment (i.e., total suspended solids), based on very high correlation coefficients (r = 0.99) obtained between suspended solids and each indicator organism for 102 water samples (35). Rheinheimer, in at least two different studies of German rivers and the Baltic Sea, found a significant relationship between turbidity and total bacterial content (32). Wuhrmann, in a review of the literature concerning river bacteriology, reported that the majority of river bacteria in free-flowing water were associated with suspended solids (39). It is not surprising that hydraulic dredging resuspends sediment-bound bacteria. The dredge William A. Thompson is capable of dredging 917 m3 of sediment per h. For every 1 m3 of sediment that is dredged, 4 m3 of overlying water is used to pump dredged material to the deposition site. This process exposes hydraulically dredged sediment to a tremendous turbulence, both at the dredge cutterhead and, to a greater extent, within the effluent distribution pipe and at the deposition site. This turbulence is sufficient to resuspend large numbers of sediment-bound bacteria, either as free bacteria or as bacteria still attached to suspended solids (epipsommic bacteria). It is probable that this turbulence is also sufficient to desorb epipsommic bacteria, even though forces holding such bacteria to particles are very strong (8, 23) and resist desorption (33). Thus, hydraulic dredging probably caused the significant effects observed in Table 2 and Fig. 2, not only because it moves and resuspends large volumes of sediment per unit time, but also because it elutes bacteria from their substrate (bacteria that otherwise would be collectively counted as an MPN index of 1 or as 1 MF colony-forming unit). Babinchak et al. (3) studied the effects of offshore dredge spoil deposition on bottom water quality and on bottom sediments at the offshore disposal site. Material (about 1.2 million m3) was removed from the Thames River by bucket dredging and transported, via hopper barges, to an offshore disposal site in Long Island Sound. MPN FC indices in the top 1 cm of sediment from the dredging site averaged 14,000 (n = 5) FC per 100 ml of sediment before dredging. Babinchak et al. (3) stated that deposition of this material had no significant effect on MPN FC indices in either bottom water samples or bottom sediment samples collected from stations in the spoil deposition area. They attributed this lack of effect to the dilution of surface, bacterialaden sediment with much greater quantities of subsurface, bacteria-free sediment. Unfortunately, sediment profiles (cores) were not analyzed to substantiate this hypothesis; they ana-

APPL. ENVIRON. MICROBIOL.

lyzed only the top 1 cm of five sediment samples collected with a Smith-McIntyre grab dredge. Also, Babinchak et al. did not state the depth of the offshore deposition area. The question of whether sediment-bound bacteria were being eluted into the water column as the dredge spoil settled to the bottom of Long Island Sound cannot be addressed because of inadequate water analyses. Thus, although Babinchak et al. stated that spoil deposition had little effect on the sediments of the deposition area, they did not examine the bacteriological water quality effects of either bucket dredging or offshore deposition of dredged material. Therefore, their data cannot be used to test our data (Table 2). I have recently obtained evidence (John A. Fish, Iowa State Conservation Commission, personal communication) documenting that a private sand-and-gravel company was dredging for sand in the vicinity of Nine Mile Island (river miles 572 to 574), site of the 1974 shigellosis outbreak in Dubuque, Iowa (6, 34). It is now impossible to retrospectively link this dredging activity to the outbreak. Moreover, water in the Nine Mile Island area was, in itself, sufficiently contaminated (17,500 FC per 100 ml) to have been considered a possible source of Shigella sonnei (34). However, in light of the data presented in this paper, it is justifiable to suggest that dredging contributed both to the area water pollution and to the waterborne shigellae. In conclusion, it should again be noted that neither turbidity nor bacteriological effects of dredging extended far downstream (Table 2 and Fig. 2). Within less than 2 km below the dredge spoil discharge area, the river had recovered from the effects of dredging. In fact, data suggest that water quality 2 km downstream and beyond became progressively better than upstream water quality (Table 2 and Fig. 2). This was probably due to natural sedimentation of suspended materials. However, it is possible that dredgesuspended particles could have served as new adsorptive surfaces in the water column, thereby increasing the rate of adsorption or flocculation (with subsequent sedimentation) of normal suspended, planktonic (unattached or epipsommic) indicator bacteria. Based on the results of this study, GREAT I made the following recommendation in their 1978 report: "If sediment samples are found to contain 100 or more fecal coliforms (mf) per gram (dry wt), every reasonable effort shall be made to alert downstream users for a distance of 2 miles of the intention to dredge." ACKNOWLEDGMENTS This work was supported by the Department of the Army, St. Paul District, Corps of Engineers (purchase order DACW376M-2345).

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WATER QUALITY AFTER HYDRAULIC DREDGING

The technical assistance of T. 0. Claflin, Maureen Plzak, Shahaireen Pellet, R. Rada, and Martin Venneman is gratefully acknowledged. I am thankful to Brenda Youngren for her helpful review of the manuscript.

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