1 Sampling - Past Climate Change

River water sampling is a fairly straightforward exercise. Choosing where to sample is often simply convenience: a bridge allows sampling from the centre of the river, and also means that the water can be carried in a car. Ideally, a river is either sampled near its mouth (and therefore the river is showing the effect of weathering the entire catchment), or at various points along its length (showing the changing effects as the river matures).
Actual sampling is not more complicated that the high-tech solution of a bucket on a rope – the water is then stored in plastic containers. The key detail is to avoid any metal, because that would contaminate the waters for the metal isotopes we will analyse them for.

At the same time as the water is collected, pH and temperature are measured, using a standard pH meter. We also determine the “total dissolved solids” (TDS) using a conductivity meter. TDS is the material in water dissolved by chemical weathering.
At the same time, we examine the amount of “total suspended sediment” (TSS), which is a measure of the amount of physical erosion. This is a measure of how much rock particles (suspended load) a river can carry. We measure this by weighing some filters before we go out into the field, and then filtering a known amount of water through the filters. These are then re-weighed afterwards, and the difference between the pre- and post-filtration weight is the amount of suspended material in the river.
Finally, we also measure alkalinity. This is the capacity of water to neutralise acid, through the presence of ions. Basically, this is what is found on the label of any bottle of mineral water: Na+, Mg2+, Ca2+, etc. In particular, the main ion is bicarbonate: HCO3-. Therefore, alkalinity is broadly a measure of the amount of bicarbonate, which is the amount of atmospheric carbon dioxide (CO2) dissolved in water.
Alkalinity is analysed by adding small amounts of dilute acid, and measuring how the pH gradually changes.

The main water sample then also needs to be filtered. This removes the suspended particles, so they can be analysed as a separate phase. Generally water is filtered at 0.2µm, although 0.45µm is also quite common. The point if filtering at such a fine level is so that when the water is analysed, it isn’t dominated by small bits of rock, which have a much higher concentration of most elements than the water itself. The filtered water is then known as the “dissolved fraction”, whereas the material retained on the filters is the “suspended fraction”. Combined with sand collected from the bed of the river at the sample site (the “bedload”), these make up the main phases of rivers: material carried in solution, in suspension, and bouncing (‘saltating’) along the river bed.
In rivers with a high suspended load (such as glacial rivers, where the physical grinding by the glacier makes a lot of fine-grained material), the filters get clogged very quickly, and have to be changed after every litre or so of water. This can make filtering very time consuming, especially when 20–30L of water is needed. Filtration needs to be conducted as soon as possible after collection, otherwise the fact that the water is no longer flowing will allow some of the suspended load to dissolve or adsorb material, changing the water chemistry.

There is one other phase in waters that we sometimes sample for: the colloidal phase. Colloids are particles that will not settle out of water, even if the water is not moving. They are often organic molecules – milk is an example of a colloidal solution: fat colloids in water. Metals such as iron, manganese or uranium are often associated with colloids. Colloids can be filtered out using a much finer filter. So fine in fact that the pore size is not determined by size, but by atomic weight. 1 Dalton (1Da) is the mass of 1 hydrogen atom, also known as 1amu (atomic mass unit). Colloids are normally filtered out at a range of 1 kDa (kilo-Dalton) to 10 kDa. Filtration at this level is even more time consuming than other filtration, and can take up to an hour per litre – again annoying if large amounts need to be filtered.

Overall, total water filtration can take many hours per sample.