UA&P EnviSci Summer 08-09 Water Toxicity Expt

Description of the Water Toxicity Experiment, 12 May 2008



The exercise is an example of the use of bioassays, i.e., tests that use live organisms.



The Payatas landfill in Quezon City is one of the Southeast Asia's biggest open landfill. It is 500 meters from the border of the La Mesa Watershed. Trash for the last 30 or so years have been dumped in this place to produce two mountains of trash covering 7 hectares and rising 7 or 8 stories above the level of the surrounding houses.



The trash produces a liquid called leachate. Leachate is a complex mixture of organic compounds and water, and possibly dissolved metals. It is toxic, and there is some danger that this liquid seeps into the groundwater. People who draw from contaminated deep wells might therefore be at risk for toxic effects.



We want to know two things. First, how do we measure the contamination? And second, knowing that, what are the risks to the populations surrounding the dump?



One way to measure contamination is to go to a chemical laboratory and have the components of the water analyzed. This gives exact amounts but has the disadvantage of being expensive and being selective only for the chemicals that are deliberately sought.



Another method is to use a bioassay. Although they have the disadvantage of not being able to identify the components of a sample, a toxic sample would cause a TOTAL toxic response on the test organism. It will simply reveal that a sample is toxic. But a bioassay is cheap, as it can make use of simple materials and test organisms that are readily sourced.



We used three test organisms: Lettuce (Lactuca sativa), Onion (Allium cepa) and a species of freshwater hydra (Hydra littoralis). Lettuce and onion were sourced from the supermarket, whereas Hydra was imported from the States and maintained at the laboratory.



To test for toxicity, we use a variation experiment; we use decreasing concentrations of the samples to check for a corresponding decrease in the toxic response. The toxic response we seek in the plants are inhibition of root growth; in hydra we look for death or abnormalities. Thus, at weaker and weaker concentrations of a toxic sample we should get longer and longer roots. The lengths should fall between the lengths obtained with clean water (negative control, or NC) and the lengths obtained with a maximal concentration of a positive control (copper for onion and zinc for lettuce).



What measure of toxicity is used and how is it obtained? Toxicity is measured by getting the average length of all roots per concentration of sample. Then all these averages are placed on a graph that shows average root length versus concentration. A line is then fit to these points using a mathematical procedure known as linear regression and the method of least sum of squares, which gives an equation for that best fit line. Then, using the equation of the line, we can compute for the concentration of sample that results in 50% inhibition of root growth, known as the median inhibitory concentration or IC50. Thus, if "clean water" gives a root length of 20 cm, and strong positive control gives 0 cm, then the IC50 of a sample would be the concentration required to give a root length of 10 cm. The IC50's can then be compared across all samples. Those with lower IC50's are the more toxic.



Now that we are able to measure toxicity, how are we going to measure risk? To be more specific, we are interested in knowing the direction in which the supposed leachate is flowing underground. Thus, we sample water from all directions around the dump (N, E, W, S) and then compare the IC50's of the samples. Given the terrain, we would expect to find more toxicity in the south wells, for example, but this will not always be the case.



Thus, this is what we must do:



1. Collect water samples from deep wells located north, east, west, and south of the Payatas dumpsite.



2. Dilute the samples and test them on lettuce, onion, and hydra.



3. After 3 days check for root growth inhibition in onion. After 4 days, check for death and abnormality in hydra. After 5 days, check for root growth inhibition in lettuce. Plot the data against concentration and determine IC50's by linear regression.