Mercury Pathways in a River and Estuary Raymond E. Cranston' and Dale E. Buckley Marine Geology, Atlantic Geoscience Center, Bedford Institute, Dartmouth, N.S., Canada
rn A method for measuring total mercury in water, suspended
particulate matter, and bottom sediments has been evaluated. Some data have been applied to a study of the geochemical pathways of mercury in a rural river and estuary system. Concentrations of mercury in the LaHave River (Nova Scotia) are related to the proximity of a small rural town. The dissipation of mercury in solution appears to be through dilution, as well as by adsorption on suspended particulate matter which raises the level in particulate matter to the range of 2.04-34.4 ppm. Bottom sediments in the LaHave River are affected by sedimentation of particulate matter containing high levels of mercury, but the mercury concentration in the bottom sediments ranges from 0.09 to 1.06 ppm. Mercury released to the natural environment from industrial waste effluents appears to be discharged mostly in the dissolved form but may be quite rapidly adsorbed as shown by analyses of suspended particulate matter and bottom sediments.
A
ccurate and precise analyses of trace elements in the marine environment have become essential as a result of the recent interest in toxic metal pollution. Part of the aim of this laboratory was to evaluate and apply an existing technique for total mercury analyses (Hatch and Ott, 1968) of water and sediment to a nonindustrial river, estuary, and open marine environment. The LaHave River system (Nova Scotia) (Figure 1) was studied as a background area because it is navigable and no lgrge amount of industrial effluent is released into it. A study of mercury pathways was undertaken to evaluate the importance of water and suspended particulate matter as a transport mechanism, because the movement, chemical form, and mobility of mercury entering the sea is important in the study of biplogical uptake (Bligh, 1970).
Field Sampling The estuary of the LaHave River is a narrow, relatively shallow reach of water that extends about 24 km inland from the headlands of the Atlantic coast of Nova Scotia. Tidal water extends inland to the residential and commercial town of Bridgewater, population of about 6500. Rural dwellings and small villages are scattered along both sides of the river from Bridgewater to the coast. A narrow navigation channel averaging 7 meters deep is maintained from Bridgewater to the mouth of the river. Water samples from the LaHave River system were taken either by pumping with a nonmetallic bilge pump and plastic hose or by a large-volume Teflon-lined water sampler (Buckley, 1970). Large volumes of water were passed through a constant-flow centrifuge on board ship to concentrate all suspended particulate matter greater than 0.2 p in diam. At each station, 4 liters of uncentrifuged water, 4 liters of centrifuged
To whom correspondence should be addressed. 274 Environmental Science & Technology
water, and the concentrated suspended particulate matter were poisoned with 10-4Msodium azide and returned to landbased laboratories. Bottom samples were obtained by grab samplers and scuba divers inserting cores by hand. Water samples from other locations were obtained by surface samplers taken in 20-liter polyethylene containers. They were centrifuged and stored in the same manner as the LaHave samples. Analytical Techniques Reagents and Standards. All reagents are certified reagent grade chemicals. Mercury standards are prepared from HgClz dissolved in deionized water (18 M k m resistivity). Stock solutions of 1 mg/ml Hg are used to prepare 0.01 pg/ml standards. A working curve is established by analyzing varying amounts of 0.1 pg/ml standard (0 to 0.20 pg Hg range). Each standard contains 50 ml of water, 20 ml of concd H2S04,and 10 ml of concd "0,. Sample Preparation. Water samples are prepared by measuring 250 ml of sample into a 500-ml Erlenmeyer flask and adding 20 ml of concd H2S04 and 10 ml of concd "0,. The solution is heated for 1 hr at 90°C. Smaller sample volumes are used for water containing mercury concentrations greater than 0.8 ppb. Sediment samples are oven-dried at 60°C and weighed into 125-ml Erlenmeyer flasks. Ten ml of water, 20 ml of concd H2S04,and 10 mi of concd HNO, are added, and the solution is heated for 1 hr at 90°C. The amount of sediment used varies from 1 gram to 10 mg depending on the concentration of mercury. Extremely high-concentration samples are diluted to maintain the absolute amount of Hg in the system below 0.2
Sample Analyses. The sample or standard is transferred to a 500-ml round-bottom aeration flask and diluted to 400 ml with deionized water. Ten ml of 377, w/v NaCl-377, w/v hydroxylamine sulfate solution is added, followed by 10 ml of 10% w/v SnS04. The aeration apparatus (Hatch and Ott, 1968) is then assembled and magnetically stirred. An atomic absorption spectrophotometer (Perkin-Elmer Model 303) is coupled with an automatic null recorder readout and a linear strip chart recorder to measure the mercury vapor absorption. All instrumental settings are those recommended in the Perkin-Elmer analytical methods book (1971). In addition to the mercury analyses described in this paper, 18 other elements and particulate organic carbon were determined in water and sediments. Results The minimum value for confident mercury analyses for this laboratory was 0.002 ppm in 1 gram of sediment or rock and 0,010 ppb in 250 ml of water. Table I contains the analyses of several standard rocks which were carried out to evaluate the precision and accuracy of the method. The significance of the difference between our results and published results is difficult to evaluate because there are few published values in the literature. Low results might be accounted for in cases where interfering elements were either oxidizing the tin(I1) more readily
Figure 1. Location map of LaJ3ave River and Estuary, Southeast coast of Nova Scotia, Canada
Sample w-1 G -2 DTS-1 GSP-1 PCC-1 AGV-1 BCR-1 Illite No.
Table 1. Standard Rock Samples Published valuesb Mean FlameHg, devialess ppma tion NAAC A A ~ AAe 0.240 0,002 0.17 0.34 0.18 0.064 0.0006 0.039 0.050 ... 0.010 0,0004 0.004 0.007 ... 0.019 0.002 0.021 0.015 . .. 0.006 0.0006 0.004 0.010 . .. 0.024 0,0000 0.004 0.015 ... 0.005 0.0005 0.007 0.005 ... 35 0.158 0.0000 ... ... ...
a Results from duplicate analyses, this laboratory. b Values for W-1 from Hatch and Ott (1968). Remaining literature values from Flanagan (1968). c Neutron activation analyses. d Atomic absorption analyses. e Flameless atomic absorption analyses.
than mercury, or amalgamation was occurring and the mercury was prevented from being vaporized. The interference due to amalgamation was not significant because the elements (Ag, Au, Pt) were present in low concentrations. The process of other elements-e.g., Fe, Mn, Cu-oxidizing the tin was minimized by the oxidizing acid medium which left these elements in an oxidized form. Even though the amount of Fe, Mn, and Cu in the standard rocks is large, no interferences appeared to be related to these elements. Since the method is very selective for mercury, it is difficult to reason how enhancement of mercury absorption could occur. Volatile organic compounds aFd dissolved gases could absorb energy in the uv region (2537A), but the standard rocks contain very little of these materials. In addition, no correlation was noted for mercury concentrations and particulate organic carbon values in the river samples. In general, there was no indication of enhancement caused by organic compounds. Tables I1 and I11 list analytical results for samples taken in
the LaHave River and estuary, and Table IV contains mercury determinations for samples from industrial areas.
Discussion LaHave River and Estuary. Measurements of total mercury in solution, in suspended particulate matter, and in bottom sediments of the LaHave River and estuary give some indication of the variability and complexity of the occurrence, transportation, and dispersal of this trace element. We have found that concentrations of mercury in solution at several stations in the river vary from 0.036 ppb to 0.380 ppb while analyses of mercury on dry suspended particulate matter gives concentrations of 3.59 to 34.4 ppm. Bottom sediments from the same locations varied from 0.09 to 1.06 ppm. Dissolved mercury content in the river suggests that an injection occurs near the center of the town of Bridgewater (Stations 2, 4) where a sewer outlet enters the river (Figures 2C and 3B). Klein and Goldberg (1970) found mercury concentrations in surface sediments up to 0.82 ppm near a Los Angeles sewer outfall. One source of mercury in sewage is from commercial chlorine bleaches which contain up to 200ppb mercury (Jonasson, 1970). The Bridgewater injection was not indicated by other dissolved trace elements (Co, Pb, Cd, Mn, Fe, Ni, Mo, Cu, Zn); however, particulate organic carbon values were highest at stations 2 and 4. The major processes involved in reducing the dissolved mercury content are adsorption on suspended particulate matter followed by subsequent flocculation and deposition, and dilution by tidal flushing. Salinity profiles for the river (Figure 2 4 show that tidal mixing occurs in the estuary up to the town of Bridgewater. The effect of these processes is the reduction of dissolved mercury content from 0.201 to 0.108 ppb between stations 2 , 4 and stations 1, 5 , respectively. The water at stations 6, 7, and 8 has a salinity of 28 parts per thousand (av), and the dissolved mercury content approaches that which was found in continental shelf seawater. The determination of mercury in suspended particulate matter is important in considering all aspects of mercury transport and mobility. Fine-grained suspended matter may adsorb and exchange ions from solution and transport them Volume 6, Number 3, March 1972 275
Table 11. LaHave River and Estuary Analyses
Station 3 (mean) 2 94 2 $4 2,4 2,4 (mean) 294 195 195 175 1,5 (mean) 195 6 6 6 6 (mean) 6 798 778 7,8 7,8 (mean) 798 9,lO 9,lO 9,lO 9,lO
Depth, meters 0 0 1.5 6.5
0.036 0.090 0.380 0.134 0.201
0.001 0.003 0.007 0.004
Hg in suspended particlate matter, ppm (dry wt) 8.05 10.9 3.93 7.12 7.32
0.175 0.057 0.091 0.108
0 002 0.002 0.017 0.007
3.59 6.39 7.58 5.85
0.01 0.01 0.13 0.04
0.057 0.056 0.058 0.057
0.003 0.003 0.003 0.003
6.32 20.2 7.05 11.2
0.02 0.06 0.02 0.03
0.054 0.076 0.046 0.059
0.002 0.002 0.002 0.002
14.0 30.2 34.4 26.2
0.03 0.06 0.07 0.05
0.071 0.088 0.058
0.002 0.002 0.003
20.2 5.12 2.04
0.04 0.01 0.01
Dissolved
Salinity,
Hg, PPb
PPt
0.0 1.9 16.2 26.6 14.9 Bottom Sediments 0.27 ppm 0 5.4 1.5 8.8 7.5 28.4 14.2 Bottom Sediments 1 .06 ppm 0 24.9 2.0 29.0 6.0 28.4 27.4 Bottom Sediments 0.12 ppm 0 26.5 2.0 28.7 10.5 29.6 28.3 Bottom Sediments 0.09 ppm 2.0 30.0 20 30.3 240 34.0 Bottom Sediments 0.14 ppm
Total particulate matter, g/l. 0,001
t
Particulate Hg, ppb 0.008 0.01 0.01 0.05 0.03
through the estuary or to deposition sites within the estuary. The significant aspects of the suspended particulate matter are therefore the size and nature of particles, the concentration in suspension, and the rate of flocculation and sedimentation. With the exception of the freshwater lense at Bridgewater, the average concentration of mercury in dried suspended particulate matter from stations in the upper estuary is liZ to 1/4 that found from stations in the lower estuary (Figure 3C). This increase in mercury concentration is probably due to increased adsorption on finer particles that remain in suspension. The general decrease in quantities of suspended particulate matter is probably due to flocculation and deposition at various sites within the estuary. The processes of flocculation
and size fractionation are related to salinity. This is illustrated by the data from stations 6, 7, and 8 where low quantities of particulate matter were found in high-salinity water, while the highest values for mercury in particulate matter were also found (Figure 2 4 B). The ratio of particulate mercury to the dissolved mercury changes from the head of the estuary at Bridgewater where it is 0.2, to the seaward end of the estuary at stations 7 and 8 where the ratio becomes 0.8. This suggests that only the finest grained particles having high adsorbed mercury persist in the higher salinity waters. To test the hypothesis that the adsorption of mercury on particulate matter is dependent on the adsorption capacity and the specific surface area of the particles, a sample of bot-
Table 111. Mercury in a Size-Fractionated Bottom Sedimenta from LaHave River Size Mean fracspecific tion in Total surface area,b sample, concn, Diam, I.( cmz/g Wt Z Hg, ppm 2 0.51 1 >60 400 20-60 600 20 0.44 12 4-20 2,000 49 0.68 44
