Comparing Different Methods of Assessing Turbidity

Abstract

In this project three different methods of testing turbidity in water will be compared, the secchi disk, spectrophotometer, and turbidity test kit. Turbidity is caused by suspended particles that scatter light passing through water. Investigating turbidity in water is very important because SAV's (Submerged Aquatic Vegetation) need light to carry out photosynthesis, and if the turbidity is too high then the SAV's get less light and they might die.

This project has two sections. The first section is, to determine if the readings from the spectrophotometer, secchi disk, and turbidity test kit correlate. From here on out this will be referred to as "Problem 1". The second section is to determine which test is the most precise, and will give the same result if repeated by another person. From here on out this will be referred to as "Problem 2".

In Problem 1, in order to establish the degree of variation within each test, fifteen different water samples were tested for turbidity using three different methods, test kit, secchi disk, and spectrophotometer. The same person did all of the tests.

In Problem 2, fifteen different people, using the three different methods, tested the same water sample in order to establish the degree of variation within each test.

Both sets of numbers were entered into a spreadsheet (Microsoft Excel 2000). For the first set of data, correlations were found between the secchi disk and the colorimeter, secchi disk and turbidity test kit, and turbidity test kit and colorimeter. The second set of data was used to determine the standard deviation within each test, turbidity, spectrophotometer, and secchi disk. Then the mean and the percentage of the mean were calculated from the standard deviation.

In conclusion this project shows that the secchi disk has a negative correlation in comparison to the other tests because it measures clarity, the opposite of turbidity. For any correlations involving the secchi disk a type of correlation was used that is for curves called a power regression. The Colorimeter and the turbidity test kit had a close correlation. The standard deviation data show that the Secchi disk is the most precise test and the turbidity test the least precise; the Colorimeter is very close to the secchi disk although not as precise. This is expected because the test kit is very dependent on perception and varies with each person and the colorimeter measures a small amount of water.

Problem

1. Do the readings from the spectrophotometer, secchi disk, and turbidity test kit correlate?
2. Which test is the most precise, and will give the same result if repeated by another person?

Precision: "The level of agreement or variability among a set of repeated measurements obtained under similar conditions. Precision is usually expressed as the standard deviation in absolute or relative terms." (EPA, "Volunteer Monitor's Guide to Quality Assurance Plans")

Background Reading

Introduction

Turbidity is caused by suspended solid matter that scatters light passing throughout the water. Too much turbidity in the water causes gill damage in fish and makes it difficult for them to find food. There are three different ways to measure turbidity. The secchi disk, a 20cm circle that is divided into quarters, which are alternately black and white. It is attached to a rope and lowered into the water and the units of measurement are centimeters, which are marked on the rope. The spectrophotometer or Colorimeter is a machine that measures the amount of light that can pass through a sample of water in a clear 10 mL tube. The turbidity test kit involves two 50 ml granulated cylinders, distilled water, Standard Turbidity Reagent, and stirring rod.

Light

Turbidity is very significant to life in the bay or any other body of water. SAV's (Submerged Aquatic Vegetation) similar to land plants need light to carryout photosynthesis, the process through which plants make food. If there is no light, there is no photosynthesis, leading to a decline in SAV's. SAV's are the start of many food chains. They are eaten by small fish, which are in turn eaten by larger fish, and these eaten by bigger fish, some of which are caught and eaten by humans. Light can reach twice as far as a secchi disk measures. For example if the secchi disk measurement is 30cm, the light can reach a depth of 60cm.

Clarity vs. Turbidity

It is thought that clarity and turbidity are too different and cannot be compared. In Water Quality Monitoring they can be used interchangeably, provided that the method used is indicated. The secchi disk is used if the bottom of the body of water is not visible, and the turbidity test or Colorimeter used if the bottom of the body of water is visible. The secchi disk measures clarity, how far down an object is visible. The Colorimeter and test kit measure turbidity, the cloudiness of the water.