Water Quality Testing

Water Quality Testing

One thing always leads to another....

I was asked by the Wildlife Trust to do some Salinity Water Tests on various water bodies as the previous volunteer had stopped doing the tests. It involves a good walk all over the nature reserve visiting all the ponds and taking Salinity readings. The results are useful to know for various reasons but recently they are being used to determine what effect the December 5th 2013 storm and tidal surge / sea breach had upon the ponds.

Since I use the blog to remind myself of my thoughts at the time when I look back in a year or two I'll mention that it takes about 12,000 steps to complete and takes about 4 hrs.

There are various types of ponds from freshwater, brackish to very brackish with some drying up in dry summers. Some are heavily influenced by the tides.

When I looked at the 2013 results for the 25 or so water bodies and compared them to the 2014 results I realised that several ponds react differently to others at different times of the year. Testing the ponds once per month, although revealing a lot of interesting information, posed more questions to me than answers. I've therefore increased the number of water bodies tested to over 50 to get a fuller picture.

Testing even more ponds then posed more questions and highlighted the fact that on a site where there are salt marshes, freshwater marshes, sand dues and meadows with man made barriers and natural as well as man made creeks and ditches connecting different parts of the site, it was a far more complicated environment than I had first thought.

What a salinity test is

A salinity test is a measurement of how much salt is in the water, at least I thought tit was. Even testing the water poses many more questions than just "what is the salinity".

For a start the results of a test can be reported as a unit of electrical conductivity, EC, which uses the unit of Siemens (that's probably where the company of the same name got it's name) but the amount of electrical conductivity is smaller than the Siemen (S) so milli or microSiemens are used (mS or uS). The test basically measures how conductive an area of water is over 1 cm of distance (the positive and negative probes are 1 cm apart). It is measuring the amount of ions in the water.

Unfortunately this EC measurement is often expressed as Parts Per Thousand (ppt) or Parts Per Million as well. This equates to milli grammes per litre and introduces another term called Total Dissolved Solids (TDS). This is the weight of salt that would be left in a bucket if you evaporated all the water off and weighed what was left.

Total Dissolved Solids

 The problem with evaporating off the water and weighing the residue is that it doesn't represent what was in the water precisely because some things that you want to measure also get lost. Evaporating water and calculating what is left is difficult and can't be done quickly or out in a field. An electrical equivalent is used instead.

Electrical Conductivity

The unit of Siemen is simply the reciprocal of the Ohm (1/Ohms). The reciprocal of the Ohm is written backwards, Mho. The problem is that different salinity testers convert the electrical conductivity to a TDS reading (ppt or ppm) in different ways.

There are various different representations of how much salt is in water.

Some, perhaps most, testers measure the conductivity and convert to a TDS value which is a representation of how much Sodium Chloride there is in water, the NaCl equivalent. They report the value in ppm.

Others convert to a TDS value which is a representation of how much Potassium Chloride there is, the KCl equivalent. Also reported as ppt.

And also other testers convert the electrical conductivity to a TDS value in ppm that represents a mixture of salts, 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride, which is the 442 representations.

A Tale of Woe

So, you could have 3 different testers that give 3 different readings.

A tester that converts 1 mS/cm into a NaCl TDS value could be 500 ppm (0.5 factor)

A tester that converts 1 mS/cm into a KCl TDS value could be 550 ppm (0.55 factor)

A tester that converts 1 mS/cm into a 442 TDS value could be 700 ppm (0.7 factor)

On top of all that, 1mS/cm electrical conductivity unit is different around the world.

The USA would convert 1 mS/cm as 500 ppm

Australia would convert 1 mS/cm as 700 ppm

Europe would convert 1mS/cm as 640 ppm

So you not only need to know what the TDS salt value represents (NaCl, 442 etc) but also which market it is aimed at.

On top of that is the temperature compensation. Water conducts electricity differently at different temperatures. From what I can gather most EC testers compensate to a temperature of 25 degrees Celcius at the rate of 2.1% per degree difference from 25 degrees C. Other machines can compensate to 20 degrees C and some machines let you assign the compensation percentage from 1% to 3%.

There is no standard at all.

Then of course you have cheap, very accurate testers  made in China (and they are accurate) and you don't know which standard mS/cm value they use (USA, Europe etc) which also doesn't help. 

I have also found that even when you know the conversion factor (0.5 for example) it isn't liner, ie, although they say that they use 0.5 the algorithm actually uses between 0.45 and 0.55 at different parts of it's full scale.

Then some manufacturers seem to use 0.71 as a linear factor.

With the temperature compensation for a reading of 500 ppm taken at 20 degrees C referenced to 25 degrees do they work out 5 x 2.1%  which is 10.5% difference and add that to 500 which equates to 552.5 ppm or do they work out the compound percentage change for 5 degrees which is:

500 + 2.1% = 510.05 ppm

510.05 + 2.1% = 521.2205 ppm

521.2205 + 2.1% = 532.1661305 ppm

532.1661305 + 2.1% = 543.341619241 ppm

543.341619241 + 2.1% = 554.751793245 ppm

The difference isn't much, 2.25 ppm, when measuring low salinity readings but if you had a value of 15,000 ppm (15 ppt) the 5 degree celcius compensation values could be 280 ppm difference.

A ppt salinity reading from one machine could be 5000 ppm while another could be 7735 ppm from another, around 50% difference for the same piece of brackish water.

The Problem

I face this exact problem. When I compare the 2013 salinity results with the 2014 results some of the results of ponds that had sea water pour into them making them very saline were 50% or so lower than in the previous year when the sea didn't pour into them. Clearly either a different machine was used to take the readings or the same machine had different settings applied. Both years results are correct and accurate and I know what settings and conversion factors were used in 2014 but what factors were used in 2013?

The difference doesn't matter much for some ponds since they were and still are fairly low salinity readings but many ponds were and still are very saline but can't be compared.

Some Detective Work is Needed

It looks like I need to try and work out what tester was used in 2013, what salt it was compared to (NaCl, KCl, 442 etc) and use the temperature readings to reverse the ppt readings back into the mS/cm electrical conductivity measurement in order to compare one year before the sea breach with the following year after the breach.....DOH!

Anglian Water

Being a bit sad and not getting out much I decided to look at my own drinking water composition as measured by Anglian Water. See http://waterquality.anglianwater.com/map.aspx 

They handily publish what the minimum and maximum values for electrical conductivity are of my drinking water. I also looked up the values for the Nature Reserve. Both my water and the Reserves tap water read identical on 2 different salinity testers. Both readings were 0.31 ppt. 

When you convert the Anglian Water EC values for both locations in ppt or convert my ppt readings into EC values (mS/cm) I find that my readings, taken with 2 accurate testers, do not fall within the range specified by Anglian Water, even when I compensate for the fact that their readings are referenced to 20 degrees C and mine are referenced to 25 degrees C. They also don't publish, not that I can find them, what type of machine they use. I can only presume they use the European value of the 1 mS/cm and not the USA one or Australian. Their readings no matter how I look at them are 25% higher than my readings.

Salinity in a pond
Another problem that I have found is that although you can take a salinity reading in a pond, if you move 20 feet along the pond and take another reading you can get a different reading, but only on some days. This tells me several things, one accuracy of the meter isn't that important unless you understand what causes this and if you do understand it the reading is wrong unless you put that reading into context. The context perhaps should be multiple readings from multiple places of the same pond on the same day within a few minutes of each test. It is caused because rain water or run off water hasn't mixed well in that location or salty water has seeped in and not mixed.   

Conclusion

I've come to the conclusion that salinity readings should only ever be done in mS/cm and not converted into TDS readings. Many more measurements need to be taken and each pond really needs a volume measurement as well as profile taken as to how salt or freshwater mixes over time since rain or high tides.

Why can't life be simple? Trying to learn something new is hard enough without these little complications.

Although I've learnt a lot I still have 2 years worth of data that I want to compare and have learnt that I can't unless I do an awful lot more work

The Future

To produce meaningful results of the salinity tests I have started to add PH tests and have started to log the tides as well as introduce the recording of the time along with the salinity results. As the salinity rises and falls I need something to compare the changes to and I presume the changes are mainly down to the tides as well as rainfall and evaporation. I have also added other information such as the life forms seen within the ponds and different vegetation as it appears.

The previous people taking the salinity readings didn't log the time or combine the readings with any other data and because it was done just once per month very little information could be gleamed from the results apart from "this pond is more or less saline that that other pond and the salinity changes throughout the year".

I want to know why it changes, what effect the rain, sun and tides have on the different ponds, at what salinity levels do the dragonflies and other insects get affected and how long does one event, such as a high tide or rain or sunny conditions, take to effect the salinity and by how much.

Environmental Science Degree

I am aiming to take a Science Degree but will need something to research and this fits the bill. To this end I took a 3rd year Environmental Science Student around the site and explained what I was doing because she is almost at the end of her degree and needs to finish her dissertation which is on the "effects of salinity with respect to the Orchids on a freshwater marsh or meadow". She turned up to see the reserve (at the wrong time of year for Orchids) with the wrong test equipment and a PH meter which was not calibrated, but to her credit she had brought some de-ionized water to do the calibration but didn't know that she needed a PH buffer to calibrate against. She also thought a salinity tester was a Calcium tester so had rented the wrong equipment and didn't know anything about an Electrical Conductivity / TDS meters even though her study depended upon the results of them. Orchids require Fungi or Mycelia to form a symbiotic relationship with the them in order to grow which she didn't know and she couldn't identify an Orchid either. She had only 4 or 5 more weeks until her whole dissertation had to be handed in complete with study data, methodology and conclusion / findings etc but apart from all that she was hopeful about passing.

Unfortunately she doesn't appear to have learnt anything in the last 3 years of University, not that corresponds to what her dissertation is about anyway, can't be bothered to look things up and admitted that when she tried to read a scientific report in her area of study that she found it rather difficult and got bored reading it, and is looking at a Masters degree next. 

When I asked how she got on her course, because I need to find a way in, she said various Universities turned her down but since she took a Maths A level (and failed with an E, or passed with an E) she could get into one of the Universities if she obtained one more A level....so she took an A level in Dance....and got accepted onto a Science based degree course.

Begs the question how good are people with degrees, what actually do they get taught and would it be worth me taking an A level in "dance" to get in?

....and I have to give her all the data that I've collected and just in case someone spots that our 2013 data doesn't correspond with the 2014 data I have to convert it for her some how.