Monitoring the Effects of Acid Rain on Freshwater Metal Chemistry Tyler Ueltschi University of Puget Sound Tacoma, WA Advisor: Anne Giblin Marine Biological Laboratory 7 MBL Street, Woods Hole, MA 2543 December 17, 213 Since the Industrial Revolution, circa 19, pollutants have been released into the atmosphere progressively lowering the ph of rain. This acidic deposition has resulted in a decline of the ph of lakes as well. Fortunately due to legislation the ph of rain is now recovering to normal levels, but it is yet to be determine whether lakes will also recover. To check in on the status of 9 lakes around Cape Cod (Duck, Dyer, Gull, Hamblin, Johns, Mares, Miles, Spectacle, and Wakeby) a lake chemistry survey was performed measuring alkalinity, ph, and concentrations of chromium, copper, iron, lead, manganese, and zinc; sediment cores were also taken from 6 of the lakes and analyzed for metal concentrations. In addition, a set of sediment cores from Johns Pond were incubated under 3 conditions, a control, sulfuric acid added, and nitric acid added, to determine the possible affects of acidic deposition on the storage of metals in lake sediments. From the general survey it was found that metal concentrations did not correlate as much with lake boat traffic as was predicted; instead the metal concentrations in the sediments correlated strongly with percent organic matter in the sediments. As for the incubations, the sediments were able to buffer the ph change more quickly than expected resulting in similar conditions between all the cores as time progressed. This buffering resulted in initial metal release from the sediments up to day 4, then very minimal metal release and some metal storage in the sediments through the end of the incubations. These results yielded insight on possible future water-sediment interactions to investigate. Introduction From the start of the Industrial Revolution pollutants have been released in higher and higher quantities into the atmosphere. This has caused a decrease in the ph of rain and an increase in dry deposition of pollutants, resulting in a significant decrease in the ph and/or alkalinity of lakes (Charles 1991). As a result of this and the general smog associated with air pollution, the Clean Air Act was passed. This legislation, which has been successful, has substantially decreased the amount of dry deposition and increased the ph of rain (NADP). 1
Sulfate deposited into lakes by acid rain in the form of H 2 SO 4 can be reduced in the sediment to form iron sulfides (e.g., FeS or FeS 2 ) and decrease the levels of acidity and sulfur in the water (Silver et. al. 22). This reduction of available iron oxides in the sediments reduces the lakes ability to store phosphates in the sediment, leading to more primary production in phosphorus limited lake systems (Silver et. al. 22). Now with a decrease in acid rain, we should see a decrease in iron sulfide storage in the sediments and a possible increase of iron in the water column or more iron oxides in the sediments compared to historical data. We also expect to a see an increase in alkalinity in all of the lakes due to an increase in the ph of rain over the last 2 years (NADP). With less sulfur in the water column to be reduced and form complexes with other metals that are more soluble under oxic conditions, such as Zn, Pb and Cu, but near constant rates of metal pollution we may see a decrease in metal storage in the sediments as depth decreases (our proxy for time) (Brick and Moore 1996). These results will be interesting to observe because a decrease in acidic deposition is generally believed to be positively influencing the environment, but it could actually decrease the storage of metals in the sediments and increase their concentrations in the water column. The affects of acid deposition on lake chemistry will be further investigated in a sediment core incubations experiment. In this portion of the research acid will be added to sets of cores in the form of sulfuric acid and nitric acid to explore the effects of added reducing power on metal storage/release along with the effects of added sulfate on metal storage. The sediments, being under anoxic conditions, should exhibit release of iron and manganese in all of the cores; however, if sulfate in the sulfuric acid incubation is reduced along with the iron oxides in the water column there may be storage of iron and 2
sulfur as pyrite. In the nitric acid incubation the lack of sulfates to be reduced may result in consistent release of iron and other metals throughout the incubation. The metal release in both of the acid treated cores should be greater than that of the control core due to the added reducing power of the acidic waters. Methods Lake Survey Water samples, 2 L each, were taken from 9 lakes in three regions on Cape Cod. The three regions were a coastal region including Duck, Dyer, and Gull Pond, an inland region including Spectacle, Hamblin, and Wakeby Pond, and an intermediate/falmouth region including Miles, Mares, and Johns Pond (Figure 1.). These water samples were analyzed for nutrients, sulfur isotopes, and alkalinity by Becky Leone, while I measured chromium, copper, iron, manganese, and zinc using an atomic adsorption spectrometer. In addition to the water samples sediment cores were taken from 6 of the lakes, Duck, Dyer, Gull, Spectacle, Miles, and Johns. These cores were taken using a pole corer with a 4 inch diameter core at about 5 meters depth in all but Johns Pond where a 3 inch diameter gravity corer was used to obtain a core at a depth of about 13 meters. The cores were then sectioned approximately every 1 cm throughout the entire depth of the core. These sections were dried and weighed to obtain a bulk density profile. The dried sediments were ground with a mortar and pestle to be used for further analyses. The portion of these analyses performed by Becky Leone included percent sulfur and percent organic matter. To estimate sedimentation rates, I measured the anthropogenically deposited lead throughout the core using a 1% nitric acid digestion. A complete 3
digestion of the sediment for measuring total metal in all of the sections was performed using concentrated nitric acid and concentrated hydrochloric acid (described in Forestner and Salomons 198). The metal concentrations were then determined using an Atomic Adsorption Spectrometer. Incubations For the incubation experiment an additional 9 cores were collected from Johns Pond at a depth of 13 meters using a 3 inch diameter gravity corer. All the cores collected contained approximately 2 cm of sediment. In addition to the cores, approximately 2 liters of water was collected. In the lab 5 liters of water was prepared for each of the 3 incubation conditions. For the control condition N2 gas was simply bubbled in the water to promote oxygen release and create anoxic conditions. For the nitric acid treatment 2 µeq of HNO3 was added to generate 2 µeq of acidity and then the water was bubbled with N2 gas. For the sulfuric acid treatment 1 µeq of H2SO4 was added to generate the same 2 µeq of acidity and then the water was bubbled with N2 gas. For each treatment 3 cores were prepared by removing the headwater and replacing it with the appropriate treated water. Initial samples and an initial ph were then taken from the 9 cores and the removed water was replaced with treated water before sealing the cores and placing them in a water bath incubation at approximately 19 C. These initial samples were analyzed for phosphate, sulfate, ammonium, alkalinity, and the metals measured in the lake survey samples. The three cores for each of the treatments were pulled consecutively at 4, 8, and 13 days. The ph of the headwater was measure and then it was analyzed for phosphate, sulfate, ammonium, alkalinity, and the metals 4
measured in the lake survey samples. The top 5 cm of the sediment was removed in 1 cm sections, dried, and analyzed for percent organic, percent sulfur, and the metals measured in the lake survey samples. Results and Discussion Lake Survey The dissolved metal concentrations did not vary significantly between the nine lakes except with regard to iron where Gull and Duck had higher levels than the others, and with zinc where Mares had higher levels with respect to the other lakes (Figure 2). Sediment cores were obtained from five lakes and analyzed for metals including lead, copper, chromium, iron, manganese, and zinc. The concentrations of metals in the top layer of sediment varied greatly (Figure 3.). Miles Pond and Johns Pond had the highest concentrations of all metals while Gull and Dyer were much lower than all of the other lakes except with regards to Chromium. Also obtained from analysis of the Gull, Johns, Miles, and Spectacle cores were metal concentrations, percent sulfur, and percent organic matter measured every 1 cm to create a complete profile of percent sulfur, percent organic, and naturally occurring metals (Figures 4-7) as well as anthropogenically deposited metals (Figures 8-11). These profiles, in general, showed a large correlation between percent organic matter and metal concentrations in the sediment. For this reason, the historically accurate lead profiles were only obtained in lakes with high percent organic matter, which included Johns and Miles Pond (Figure 9 and 1). For the Miles Pond profile the lead concentrations dropped to preindustrial revolution levels at a depth of about 12 cm. 5
When examining the lake chemistry survey results it was difficult to distinguish between lakes based simply on the metal concentrations in the water column (Figure 1.). It was expected that lakes with higher amounts of boat traffic and larger amounts of recreational use would have higher amounts of metal pollution in the water column. These metals associated with boat use, mostly chromium, copper, and zinc, were not significantly higher in Johns Pond or Gull Pond where recreational use is the highest. This indicates that there either isn t a significant amount of pollution input to the lakes or this pollution is being stored in the sediment. For this reason sediment cores, from the lakes at which they could be obtained, were examined. The sediments, sectioned and acid digested, showed much more variation in metal concentrations near the surface of the sediment (Figure 2.). However, the high traffic lakes, Johns and Gull, did not necessarily have the highest metal concentrations at the surface of their sediments. Miles Pond actually had higher concentrations of zinc than Johns Pond, with similar concentrations of copper in both and higher chromium in Johns Pond than Miles. This originally raised concern because it was believed that Miles was a rather pristine lake with little to no recreation use so it should not have high concentrations of anthropogenically deposited metals. But after conversation with John Hobbie, it was noted that the lake had an ice manufacture along its shore historically and was most likely the source of the zinc and copper pollution. It was also unexpected that Gull Pond would have relatively low concentrations of anthropogenic metals because it has been used for recreation heavily in the past. 6
To determine how metal content may have been influenced by other sediment parameters, we plotted the percent organic matter in the top section of the cores against the total concentration of the anthropogenic metals measured (zinc, copper, and chromium) (Figure 12.). By plotting these two against each other and using a linear fit, a correlation of.998 was found indicating that the amount of metal held within the sediment is directly related to the amount of organic matter in the sediment. So to accurately compare metal concentrations in these lakes, cores from deeper parts (presumably higher in organic content) of Dyer, Gull, and Spectacle would need to be collected. Cores with higher organic content throughout the depth of the core would also allow us to develop better depth profiles for the metals which were for the most part inconclusive in terms of dating based on lead deposition in all but Miles Pond. Incubations For the incubation experiment the goal was to determine how acid rain or ph drop in a lake affected metal release or storage in the sediments. Johns Pond cores were used for the experiment because of the high concentrations of metals and the relatively uniform core. The initial spike of acidity to the two sets of treated cores resulted in a drop of the ph from 6.7 to 5. (Figure 13). While this drop in ph is substantial it is possible that a similar drop could occur in nature due to acid rain over time. The water in the incubations, previously bubbled with N2 gas, was presumably anoxic producing good reducing conditions in the cores. After 4 days the first core was pulled from the incubation and analyzed. The ph of all of the cores had converged around 6.-6.3 at this point so the only difference in the incubations 7
after this time point would be the added sulfate or nitrate. This convergence of ph was somewhat unexpected and if the experiment was repeated I would either add more acid at the initial time point to keep the ph lower longer or add smaller amounts throughout the incubations to maintain a more consistent ph. The sulfate concentrations in all of the cores were consistent through day 4 and indicated that there weren t any considerable amounts of sulfur storage in the sediment. At day 4 the sulfuric acid treatment had the largest amount of metal release for all but chromium, which decreased in concentration for all of the treatments (Figure 14 and 15). This was the expected result for the sulfuric acid treatment especially since there was no sulfur storage that could promote iron or other metal storage. For the last two time points the ph and sulfate concentration stayed consistent in all of the cores, while the phosphate concentration increased overall in all of the cores (Figure 13.). It is tough to draw solid conclusions based on the behavior of individual cores, and it was odd that the phosphate showed the highest release in the control core where the least amount of phosphorous should have been released. The metal concentration for these last two time points was also interesting with the sulfuric acid treatment showing a spike in iron at 8 days but a large drop at 13 days without a large corresponding drop in the concentration of sulfate. As for the iron concentrations in the rest of the cores, the control showed the same pattern as the sulfuric treatment (Figure 14.). The nitric acid treatment followed the expected behavior of slow continual release, but it is hard to determine if this is because of discrepancies in the individual cores or actually attributed to continual iron release from the sediments. In all of the cores the manganese concentrations remained 8
consistent through day 13 with a slight spike at day 8 in the sulfuric treatment and a slight depression at day 8 in the nitric treatment (Figure 14.). For the rest of the metals over the last two time points, copper remained fairly consistent in all of the cores while zinc had a depression at day 8 in the treated cores then increased at day 13 with the control consistent throughout and chromium increasing slightly over all of the cores at day 13 (Figure 15.). The results for copper and chromium show such low variation that the fluxes are inconclusive. The results for zinc, which show larger amounts of change, are odd because they drop considerably at day 8 but none of the other measure concentrations showed similar behavior that would suggest zinc storage in the sediments so it is hard to attribute this to a result of the treatments. Overall it was difficult to draw conclusions from the incubation experiment with only one core per treatment and time point. In the future I would like to repeat this experiment in a similar fashion with replicate cores, more time points, and longer incubations. Acknowledgements I would like to acknowledge Anne Giblin for her help as my advisor on the project, providing considerable background knowledge to the project as well as supplies and equipment. Also Becky Leone as my co-research assisting with all of the sample collection and taking care of the non-metal analyses. And the course assistants, Fiona Jevon, Alice Carter, Sarah Nalven along with Rich McHorney and the rest of the SES MBL staff/researchers. References Brick, C. M., Moore, J. N. 1996 Diel Variation of Trace Metals in the Upper Clark Fork River. Montana Environmental Science & Technology, pp. 1953-196. 9
Charles D.F. (ed.) 1991. Acidic Deposition and Aquatic Ecosystems. Springer- Verlag, NY, pp. 421-467. Forstner, U. and W, Salomons. 198. Trace metal analysis on polluted sediments. Environmental Technology Letters 1 pp. 494-55. Gibbs, M. M, Hickey, C. W, Özkundakci, D. 211. Sustainability Assessment And Comparison Of Efficacy Of Four P-Inactivation Agents For Managing Internal Phosphorus Loads In Lakes: Sediment Incubations, Hydrobiologia pp. 253-275. Macleod, C. K. et. al. 212. Measuring Hypoxia Induced Metal Release From Highly Contaminated Estuarine Sediments During A 4 Day Laboratory Incubation. Experiment Science Of The Total Environment pp. 229-237. National Atmospheric Deposition Program. October 3, 213. http://nadp.sws.uiuc.edu/. Website. Siver, P. A, R, Ricard, R, Goodwin, A. E, Giblin. 22 Estimating Historical Inlake Alkalinity Generation from Sulfate Reduction and its Relationship to Lake Chemistry as Inferred from Algal Microfossils, Journal of Paleolimnology pp. 179-197. 1
Inland: Spectacle, Hamblin, Wakeby Falmouth: Johns, Mares, Miles Coastal: Duck, Gull, Dyer Figure 1. A map of Cape Cod with the lakes surveyed labeled and listed by region.
Concentration (mg/l).5.45.4.35.3 Copper Zinc Chromium Manganese Iron.25.2.15.1.5 John Wakeby Spectacle Hamblin Dyer Duck Gull Miles Mares Figure 2. A bar graph of the metal concentrations in the water column from the nine lakes surveyed on Cape Cod.
Concentration (µg/g) 1 Zinc Manganese 1 Copper Chromium Iron (mg/g) 1 1 Miles Johns Spectacle Gull Dyer Figure 3. A bar graph of the metal concentrations in the top layer of sediment from the five lakes cored on Cape Cod.
Depth (cm) Depth (cm) 2 4 % Organic.1.2.3.4.5.6.7 % Sulfur.2.4.6.8.1.12 6 8 1 12 14 16 2 4 Manganese (µg/g) 2 4 6 8 1 12 14 Iron (mg/g).5 1 1.5 2 6 8 1 12 14 16 Figure 4. The depth profiles of Gull Pond for % organic matter, % sulfur, manganese, and iron.
Depth (cm) Depth (cm) 2 4 % Organic 5 1 15 2 25 3 % Sulfur.1.2.3.4.5 6 8 1 12 14 16 2 4 Manganese (µg/g) 5 1 15 2 25 Iron (mg/g) 1 2 3 4 5 6 7 8 6 8 1 12 14 16 Figure 5. The depth profiles of Johns Pond for % organic matter, % sulfur, manganese, and iron.
Depth (cm) Depth (cm) % Organic 2 4 6 8 1 % Sulfur.2.4.6.8 1 1.2 1.4 1.6 5 1 15 2 25 Manganese (µg/g) 2 4 6 8 1 12 Iron (mg/g) 2 4 6 8 1 12 14 5 1 15 2 25 Figure 6. The depth profiles of Miles Pond for % organic matter, % sulfur, manganese, and iron.
Depth (cm) Depth (cm) % Organic 5 1 15 2 25 3 Sulfur %.1.2.3.4.5 5 1 15 2 25 Manganese (µg/g) 2 4 6 8 1 Iron (mg/g) 1 2 3 4 5 6 7 5 1 15 2 25 Figure 7. The depth profiles of Spectacle Pond for % organic matter, % sulfur, manganese, and iron.
Depth (cm) Depth (cm) 2 4 Lead (µg/g).5 1 1.5 2 2.5 3 3.5 Chromium (µg/g) 5 1 15 2 6 8 1 12 14 16 2 4 Zinc (µg/g) 5 1 15 2 25 3 35 4 Copper (µg/g) 2 4 6 8 1 12 6 8 1 12 14 16 Figure 8. The depth profiles of Gull Pond for lead, chromium, zinc, and copper.
Depth (cm) Depth (cm) 2 4 Lead (µg/g) 2 4 6 8 1 12 14 16 Chromium (µg/g) 1 2 3 4 5 6 7 6 8 1 12 14 16 2 4 Zinc (µg/g) 5 1 15 2 Copper (µg/g) 1 2 3 4 5 6 7 8 6 8 1 12 14 16 Figure 9. The depth profiles of Johns Pond for lead, chromium, zinc, and copper.
Depth (cm) Depth (cm) Lead (µg/g) 5 1 15 2 Chromium (µg/g) 5 1 15 2 25 3 35 4 5 1 15 2 25 Zinc (µg/g) 1 2 3 4 5 6 7 Copper (µg/g) 2 4 6 8 1 12 5 1 15 2 25 Figure 1. The depth profiles of Miles Pond for lead, chromium, zinc, and copper.
Depth (cm) Depth (cm) Lead (µg/g) 1 2 3 4 5 6 7 Chromium (µg/g) 2 4 6 8 1 12 14 5 1 15 2 25 Zinc (µg/g) 2 4 6 8 1 12 Copper (µg/g) 5 1 15 2 5 1 15 2 25 Figure 11. The depth profiles of Spectacle Pond for lead, chromium, zinc, and copper.
Total Anthropogenic Metals (µg/g) 8 7 6 5 4 3 y = 16.547x + 35.887 R² =.9982 2 1 1 2 3 4 5 % Organic Figure 12. A scatter plot of the total anthropogenic metal concentration from the top 1 cm of sediment in each of the lakes cored plotted against the percent organic matter in these sediments with a best fit line.
Sulfate (um) Phosphate (um) ph 7. 6.5 ph 6. 5.5 5. Control S N 4.5 4. 1.8 1.6 1.4 1.2 1..8.6.4.2. 2 4 6 8 1 12 14 Phosphate 2 4 6 8 1 12 14 Control S N 23 21 19 17 15 13 11 9 7 5 Sulfate 2 4 6 8 1 12 14 Time (Days) Control S N Figure 13. Scatter plots for the incubation experiment for the ph, concentration of phosphate, and concentration of sulfate in the water column at time, 4, 8, and 13 days.
Manganese (mg/l) Iron (mg/l) Iron.9.8.7.6.5.4.3 Control Nitric Sulfuric.2.1 2 4 6 8 1 12 14 Manganese.35.3.25.2.15.1 Control Nitric Sulfuric.5 2 4 6 8 1 12 14 Time (days) Figure 14. Scatter plots for the incubation experiment for the concentration of iron and manganese in the water column at time, 4, 8, and 13 days.
Zinc (mg/l) Chromium (mg/l) Copper (mg/l) Copper.1.8.6.4.2 Control Nitric Sulfuric 2 4 6 8 1 12 14 Chromium.135.13.125.12.115 Control Nitric Sulfuric.11 2 4 6 8 1 12 14.6.5.4.3.2.1 Zinc 2 4 6 8 1 12 14 Time (days) Control Nitric Sulfuric Figure 15. Scatter plots for the incubation experiment for the concentration of copper, chromium, and zinc in the water column at time, 4, 8, and 13 days.