Professor Susan Walden, Glasgow University
Chris - If you’ve ever turned on a tap for a glass of cool clear water and watched brown water coming out instead, then you’ll know that it’s less than appealing.
Susan Waldron - The brown colour occurs when there is a high concentration of dissolved organic carbon in the sample and the higher the concentration of dissolved organic carbon the deeper the colour. That dissolved organic carbon is coming from the soils that are in the catchment around about the drainage system, they break down and produce material that goes into solution and then when we have movement of water from the catchment soil, such as under a rainfall event, we have movement of that carbon into the catchment water.
Sue Nelson - Now you've got a couple of samples of water here ranging in colour. We've got a bottle of mineral water on the side there, so that's very clear to, I suppose there's no way of putting this, it could be urine samples, I suppose.
Susan Waldron - We like to think of them closer to whisky but, yes, the darker the colour the higher the concentration generally of dissolved organic carbon and in river systems we can see concentrations that can be up to 16 mg/litre carbon and they can really be quite dark brown samples.
Sue Nelson - Has there been an increase in the amount of carbon and brown water that you get nowadays?
Susan Waldron - Within the UK there are quite a large group of scientists who understand very well the processes by which carbon is transmitted into a catchment drainage systems and what they have observed is that there has been an increase in the amount of carbon that is going into water over Europe, the UK, parts of North America and this has happened over quite a long timescale, about 20 or 30 years. There are multiple competing hypotheses for this but the one that is most commonly accepted is that actually as we clean up our atmospheres then we reduce the amount of acid deposition that there is, it promotes the breakdown of organic material and produces this dissolved organic carbon that can then go into the river systems. So, effectively, as the atmosphere's become cleaner the water can become browner.
Sue Nelson - That seems counter intuitive doesn't it?
Susan Waldron - Yes, but the sulphate deposition inhibited the production of dissolved organic carbon.
Sue Nelson - Sorry, what do you mean by that?
Susan Waldron - Sulphate deposition is your acid drain. Okay, so as we clean up the acid deposition that is coming from large scale industrial manufacturing, we're cleaning up the atmosphere and we're not depositing so much sulphate on our soils and we're increasing the amount of carbon that is being produced as dissolved organic carbon.
Sue Nelson - But water companies, I am assuming, though, must spend extra money trying to make brown water clear?
Susan Waldron - That's correct because aesthetically people don't like to drink brown water, it looks dirty. We associate the brown colour with soils and therefore we think that our waters are contaminated with soils and it could be that if the water is not purified of the material that gives it the colour it may not also be purified of other components. So this is understandably why people don't want to drink coloured water. So, therefore, the fact that our waters are becoming more coloured presents a problem for the water companies because they have to invest more in cleaning up the water and they need to also understand how the carbon is arriving at the water purification plant. So, is it coming in at a continuous low level which is just increasing or is it coming in in spikes and can their equipment actually cope with this increase in carbon concentration if it comes in in a spike or not.
Sue Nelson - And what are you discovering so far?
Susan Waldron - That land use-change can affect the increase in carbon concentrations. We know well that there is a very strong hydrological response to movement of carbon into river systems and that there is a seasonable component as well. So, we understand that at the end of the summer when we've had higher productivity in the landscapes and then subsequent breakdown of this organic matter, and we're supposedly into a wetter period when the catchments start to wet up again in the autumn time, when we have heavier rainfall, then that is when we have the largest amount of carbon moved into the catchment. But it is very interesting because we are potentially moving to a situation where we will have different levels of productivity as temperature regimes change, and as has been apparent over the past few months where we have had the wettest quarters since records began recently, the time period when water was being delivered to the catchment also changes. So what we don't understand yet is how carbon will be delivered to the catchment under a changing climate and that's very important because the companies need to be able to understand better how to manage their resources in purifying their water as best as possible. So we're taking our knowledge about what we know the processes that generate carbon in the catchments, how it is delivered to the catchment and then trying to understand how the changes might occur under projected climate change.
Interesting notes about dissolved organic matter (carbon).