Nailing the hyporheic zone: Rusting metal rods as a proxy for the depth of the oxic zone in hyporheic sediments

Changing climatic drivers significantly endanger the function of the hyporheic zone (HZ). Increased temperature leads to an increase in respiration and expansion of anoxic areas in river sediments. Under conditions of low stream discharge (with high proportion of groundwater) reduced substances enter the upper parts of the HZ and surface water. Contact with dissolved oxygen (DO) leads to formation of areas of preferential oxidation of reduced substances (e.g. iron precipitation and clogging of pore spaces, nitrification).  Thus oxygen levels in the hyporhic zone are an important parameter for understanding biogeochemical cycles in the stream sediments. However, oxygen measurements are time consuming, prone to error and instruments are expensive.

In this work we hypothesize that iron nails can be used as a very simple and inexpensive tool for mapping the depth of oxygen penetration into hyporheic sediments. The experiments compared iron oxidation of nails inserted in a laboratory scale sand tank model for hyporheic flow as well as a riffle-pool sequence in northern Bavaria, Germany. We have combined this with a mathematical model that simulates flow and biogeochemical reactions in the subsurface and with direct oxygen measurements in the sediment.

Oxygen measurements showed that oxygen concentrations decrease from over 8 mg l^-1 to less than 1 mg l^-1 in the first 15 cm of the tank model. This agreed well with the mathematical model, which predicted the decrease of oxygen to approximately the same depth. The nails showed a clear rust (iron oxide) crust down to sub-oxic DO levels (at ~11 cm) while below this we observed a black precipitate indicating reducing conditions. In some cases, we found an area between the oxic and anoxic zones where there were no obvious signs of reaction on the nails surface. The mathematic model indicated that the water residence time in the oxic part of the tank was ~5 h while it took up to 19 h to pass through the anoxic zone and out of the tank.

The field results confirms that rust on the nails can be used to indicate oxic depths, but with a somewhat more complex pattern along the riffle-pool sequence.

Iron oxidation and associated rust on the metal nails gives us only qualitative data on the presence or absence of Oxygen. Quantitative data may be derived by determining the iron mineral phases on the nails (using e.g. Raman spectroscopy). Overall this appears to be a very inexpensive method for gaining high spatial resolution information of oxygen levels in the hyporheic zone, which is important for stream ecosystems as well as biogeochemistry.