If you are a California native, you would have heard of The Salton Sea. The Sea is actually a lake located in the Imperial and Coachella Valleys that was accidentally made (Fig. 1). A massive flood from the Colorado River broke through a canal nearby and the water from the Colorado River flowed for 18 months into a once dry, desert basin. Today, the only bodies of water that feeds the Sea are from the rivers nearby and agricultural runoff and sewage systems from the nearby community. The Rivers (New River and Alamo River) themselves are impacted by wastes from urban and agriculture land use. Since the last decade, risks of contaminants in the water and sediment of Salton Sea has been of concern. From previous studies, volatile (and semi volatile) compounds and metals have been examined and found in the Sea. In our lab’s study, we set out to analyze Salton Sea’s sediment.
Figure 1. Map of the Salton Sea.
The Chemical Analysis of Sediments lab’s purpose was to learn how to analyze the sediment sample, chemically and physically, in order to find the organic contents within it. There were a few parts to this lab:
- Gas Chromatography-Mass Spectrometry
- Laser Particle Scattering Spectrometry
- Measuring carbonate and organic content
Preparing the Sediment
The sediment from the Sea was dried for a few days and using the QuEChERS method (an extraction method) pesticides that were in the sediment were extracted. This liquid extract was used for the GC-MS runs. For the particle analysis via light scattering, 2 grams of sediment and 2 mL of water was made in a tube and this was the sample to be analyzed.
Splitless, untargeted run of the sediment extract was done in SCAN mode and it looked for things that were within the 45 – 475 m/z range. The mass spec data was analyzed and the peaks were assigned to different compounds that were matched from the GC-MS library. After this was done, another GC-MS run was done but this time using the SIM SCAN method which looks for specific target compounds. Whatever pesticide we were looking for, the mass ranges (at least 3) for that specific pesticide was looked for. The following pesticides have been identified in the Sea’s sediment:
Laser Particle Scattering Spectrometry
CILAS particle size analyzer machine was used for the light scattering experiment. Scans were done for the sediment sample in order to analyze the % clay, silt and sand within the sediment sample.
Measuring carbonate & organic content
To measure the bicarbonate content in the sediment, the dried sample was vacuum filtrated and treated with hydrochloric acid. Hydrochloric acid turns the calcium carbonate in the sediment to carbonic acid and all of the carbonate has been converted to gas phase. The sample was dried and re-weighed after this treatment. The mass lost was due to the loss of calcium carbonate. The % calcium carbonate was calculated from the mass lost.
To measure the organic content in the sediment, the dried sediment sample after the HCl treatment was heated inside a crucible until glowing red using a high temperature bunsen burner. This method is also known as “loss on ignition” or LOI. Once the sample was cooled, the sample was weighed. The mass lost was due to the oxidation of organic compounds in the sediment to carbon dioxide.
From the GC-MS analysis, we found that there were only 3 pesticides in the sediment: Chlorpyrifos, Permethrin, and Tetraconazole. No detection of DDT! From the laser spectrometry, we found that there was 1.49% clay, 25.52% silt, and 72.99% sand in the sediment. After analyzing calcium carbonate and organic content in the sediment, we found that the sediment contained 5.72% calcium carbonate and 1.84% organics overall.
From our studies with the Salton Sea sediment, we have concluded that there are lots of chemicals in the sediment that shouldn’t be there. Overall, the assessment results suggested potential ecological risk in the Sea’s sediment and a solution is needed. Remediation strategies are needed given how important the Sea is to the communities nearby.