Bioavailable Heavy Metal Sediment
Loading In Laurel Creek

R. John Kominar,
Department of Chemistry,
Wilfrid Laurier University,
Waterloo, ON

Introduction

Contamination of aquatic systems by heavy metals and other pollutants is a problem in society because the environment is often used as a receptacle for waste products generated in the economy.

Stock pollutants such as heavy metals are not broken down and persist in the environment. Heavy metals are contained in four reservoirs in an aquatic environment, namely, the surface water, the pore waters, the suspended sediment, and the bottom sediment.

Metal concentrations in surface waters can fluctuate by several orders of magnitude over short time intervals depending on the conditions . Heavy metals in aquatic systems tend to accumulate in bottom sediments and can be released by various processes of remobilisation. The concentrations in sediments do not fluctuate to the same degree as in surface waters . Thus sediments of aquatic systems are usually studied when studying the quality of a water system. The most mobile fraction of the sediment is that of the < 63 Ám grain size. This fraction also contains proportionately more sorbed metal/gm of sediment due to its larger surface area. While both bioavailable and total concentrations of metals are typically studied, recently there has been more focus on bioavailable metal forms because these are the ones that have the greatest environmental impact.

Objectives

The objectives of this project were to:
  1. Determine the bioavailable concentration and spatial distribution of selected heavy metals in the < 63 Ám grain size fraction of bottom sediments collected by different methods.

  2. Examine temporal variability in sediment metal levels in Laurel Creek . Two studies are reported here, one conducted on October 7, 1992, the other on Oct.29, Nov.3, and Dec. 10,1994.

  3. Compare the concentrations of metals found in Laurel Creek sediments to Ontario Ministry of the Environment (OME) Sediment Quality Guidelines and the American Environmental Protection Agency (EPA) Guidelines.

Sampling Sites

  1. Waterloo-Wilmot Line ~ 2 km West of Erbsville
  2. University Ave. & Westmount Rd.
  3. Waterloo Park
  4. City Hall - Waterloo
  5. Moses Springer Park
  6. Hillside Park - upstream of Forwell Creek
  7. Hillside Park - Downstream of Forwell Creek
  8. Near the confluence with the Grand River

Metals Studied

The metals studied were iron (Fe), manganese (Mn), nickel (Ni), copper (Cu), lead (Pb), cadmium (Cd), and chromium (Cr). In terms of toxicity and accessibility Fe and Mn, which are present in higher concentrations are considered noncritical, while the trace metals Ni, Cu, Pb, Cd, and Cr are toxic and relatively accessible.

Methodology

Sediment Sampling

Samples were collected in three ways. Surface samples were obtained by scooping the top one to two centimetres of sediment into plastic tubes. Core samples, varying from seven to seventeen centimetres in depth, were collected with plastic core-sampling tubes. Bottom Sweep samples were collected by sweeping the bottom sediment with a wide broom and collecting the water-sediment suspension in twenty litre pails.

Sample Preparation

The core samples were divided according to the horizons present. The bottom sweep samples were allowed to settle and the water poured off. All samples were oven dried at 70 - 1000 C, gently crushed with a rolling pin to disaggregate the samples but not break down the grains themselves, and sieved to collect the less than 63 micron grain size.

Determination of Bioavailable Metals

One gram samples of sediment were treated with 25 mL of 0.5 M HCl, shaken for 16 hours, centrifuged for five minutes at 5000 rpm, filtered through 0.45 micron fibre filters, and the solutions analyzed for metals by flame atomic absorption spectroscopy. The 0.5 M HCl extraction removes sorbed metals but does not remove metals from the matrix sediment particles themselves.

Definition Of Pollution Levels

The results are compared to Ontario Ministry of the Environment (OME) Guidelines For the Protection and Management of Aquatic Sediment Quality in Ontario (1992), and United States Environmental Protection Agency (EPA) criteria. The OME Lowest Effect Level (LEL) represents a level of sediment contaminant tolerated by most benthic organisms. The OME Severe Effect Level (SEL) represents a level of pollution causing pronounced disturbances in most of the sediment dwelling community. The U.S. EPA Heavily Polluted Category (HPC) refers to regulations governing the discharge of dredged or filled material in navigable waters.

Results And Discussion

Attention should be drawn to the importance of obtaining representative samples when interpreting results of tests of the type reported here. Although the confidence in the amounts of metal determined in individual samples is high, the homogeneity of samples collected from an individual site may vary substantially and often did in this case. This must be taken into account when evaluating the significance of the results. The results of the 1994 surface samples presented here, for example, represent the average of values for seven individual samples collected from a small area at each site. While the average of this many determinations probably presents a fairly accurate picture of the metal concentration at a particular site, single samples sometimes varied by as much as 50% from this average value, with individual samples in a few cases varying from each other by as much as a factor of two. This is a convincing argument for collecting many samples at each site and not drawing conclusions on the basis of one or two results.

Table 1 presents a summary of the average concentrations (mg/kg) of metals in surface samples taken from the various Laurel Creek sites in 1992 and 1994 and compares them to the various pollution guidelines. These data are depicted graphically on Graph 1 (a - g).

Site 1, the rural agricultural site, showed the highest concentrations of iron and manganese. This is not unexpected for an agricultural area. The concentrations of all five trace metals were the lowest at this site.

Site 4, in the centre of the city, showed the highest concentrations of all trace metals, with the 1992 concentration of copper exceeding the Severe Effect Level, and the 1994 reading very close to the Severe Effect Level. The next highest concentrations were at the confluence with the Grand River.

Table 2 is a summary of the metals falling into each of the pollution categories chosen for comparison. With five of the seven metals tested at concentrations above the lowest effect level and two metals, Manganese at the rural site and Copper in the centre of the city, above the severe effect level there should be some concern about metals in Laurel Creek, and appropriate action considered.

The core samples analyzed in 1992 indicated that the concentrations of Cd, Cr, Ni, Cu, and Pb were higher in samples with higher organic content. Since the organic content of the Laurel Creek sediments tested is quite high there is the potential for Laurel Creek being a significant contributor of contaminants to the Grand River which eventually empties into Lake Erie. The copper, cadmium, and lead concentrations in the Laurel Creek sediments were substantially above background levels of these elements in Great Lakes sediments.

An encouraging result from the 1992 core sampling was that, with the exception of Copper, the concentrations of metals in the upper layers was found to be less than in the lower or deeper layers. This could guardedly be interpreted as indicating that for the most part metal pollution has been decreasing in recent years.

Acknowledgement

The author wishes to acknowledge Mr. Brian Smida and Mr. Jeffrey Newman who did their senior research projects under his guidance and assisted in much of the sample gathering and analysis. Thank you also to Dr. Mike Stone, Department of Planning, University of Waterloo, for his assistance and guidance with the experimental design and methodology.


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