Sampling and Statistics of Water Quality Flashcards
(14 cards)
How are companies responsible for their sampling?
Responsible for continuous monitoring of quality of their inputs and outputs by the environmental agency.
When is a grab sample sufficient?
Okay for a uniform source or for a spot check by the EA
What is a composite sample and when is it needed?
Collected over time and space and is needed for raw water and wastewater due to their high variability in quality and quantity.
Collect samples and flow at same time
What is important when designing sampling programmes?
Selecting the objectives very carefully to measure the correct things. Must specify a tolerable level of uncertainty.
What are the 3 main ways of analysing samples?
Explain each
Gravimetric - weighing solids
Volumetric - measure volume of substance required to cause a certain reaction
Colourmetric - measure colour
Why is analysis of samples difficult?
Measuring samples with very low concentrations
What does a histogram show?
A plot of concentration against the number of samples in each concentration range.
Shows number of samples in excess of a specific value.
How can a distribution be drawn and what does it show?
Can be drawn from the histogram data if we get enough data.
Can tell us how many samples exceed a particular value and how which time is spent there.
What is a normal distribution?
When distribution is symmetrical and average concentration is at peak.
What distribution does water quality tend to be in and why?
Log - normal since high concentrations are usually further from middle than low ones which suggests an unsymmetrical or long tailed distribution.
Why does the EA assume a distribution shape when looking at data?
- Reduces error in estimates and summary statistics. Can detect small changes in water quality with the input of extra knowledge from the distribution.
- Obtain rough estimates of summary statistics such as 90th percentiles
- Saves time in calculating permit conditions to meet environmental targets
How do we compare the results to check water quality is good?
Compare against standards which are expressed as summary statistics
Where is the biggest source of error in the summary statistics and why?
Number of samples.
Limited samples can cause a lottery effect with quality changing potentially every second.
What errors are less important?
Errors in subsequent chemical analysis
