Coupling Groundwater Residence Times with Geochemical Tracers

Introduction

Understanding controls on chemistry in streams has been a key focus of research in Critical Zone science for some time now. While a growing understanding has emerged pointing towards water-rock interactions as a primary control on stream water chemistry, both the fields of hydrology and low temperature geochemistry have different reasons as to why.

In general, for hydrology, groundwater ages tend to correlate with differing chemical compositions. The longer the age, the more the water looks like the bedrock, while younger ages tend to look more like soil waters. A key implication of this is the coupling of groundwater ages with depth (though it does become more complicated than just that). As for the geochemical perspective, chemical weathering of bedrock is a reaction that takes time to reach equilibrium with the surrounding water. If groundwater interacts with bedrock for enough time, then the groundwater chemistry will reflect the chemistry of the minerals weathering into the water. However, if the timescale of the interaction is short, then the minerals in the bedrock will not fully reach equilibrium.

Here, we are attempting to look at geochemical tracers and groundwater ages to see how well both correlate with regards to controlling observable stream chemistry.

Sagehen Creek

The field site where we are conducting this study is a small (~27² km) montane catchement located in the central Sierra Nevada mountain range. Sagehen Creek has been a reserve since about the mid 1950s. There’s been a particular focus on entomology, hydrology, and ornithology research in the basin. It has a mediterranean climate that receives about 850mm of annual precipitation, of which 80% falls as snow. The predominant lithology of the basin is basaltic andesite that overlays granitic layer. Work in the basin has shown that stream flow is largely groundwater dominated.

The reason we are interested in Sagehen Creek is due to the previous work of Dr. Laura Rademacher, who has done extensive work to constrain groundwater ages of the stream and springs in the basin. Her work, and studies that built of her work has shown that the apparent groundwater ages that support stream flow are decadal, which implies deep flow paths. It has also shown that the ages (or groundwater residence times) of the springs are increasingly aging. This implies that shallow flow paths are contributing less water to the springs, and that deeper flow paths are contributing more as a result.

This groundwork of constraining the ages of groundwater across the basin is huge as it provides deep insight into the flow paths water is taking before entering Sagehen creek or the springs

Where I come in, is applying a series of geochemical tracers to compliment the apparent ages that have been well studied thus far.

Geochemical Tracers

What are geochemical tracers? Simply put, they are tools used to track the weathering of rocks over time. Hydrologists tend to use concentrations of dissolved cations in water that are found commonly in minerals to get an idea of weathering. Some examples of this are Mg²⁺, Ca²⁺, Na⁺, and SiO₂ (whats considered mobile Si, a base component of minerals).

While these tracers are useful to get an idea of both flow paths and the breakdown of rocks overtime, they do not show sensitivity to how long it takes for the rock/minerals to break down, or even how long the water has to be in contact with the rock to reach observed concentrations.

Thankfully, there are a set of tracers that are time sensitive and can provide insight to the breakdown of bedrock and the formation of new rocks (clays). These tracers are silicon isotopes (reffered to as $\delta$³⁰Si), and germanium/silicon ratios. Silicon has 3 stable isotopes (or forms), ²⁸Si, ²⁹Si, and ³⁰Si. When bedrock weathers, it releases these forms of Si that were stuck into the rock into the water. If the $\delta$³⁰Si of the water looked exactly like that of the rock, then we know the water had time to reach equlibrium with the rock. However, $\delta$³⁰Si is also sensitive to the formation of new rocks. If new clay is forming, it takes up a lot more ²⁸Si than the heavier ²⁹Si or ³⁰Si. This makes the $\delta$³⁰Si of the water much heavier. It doesn’t take long for stream water $\delta$³⁰Si to reach equilibrium again with the newly forming clay rocks. What does take a lot longer to reach equilibrium though, is Ge/Si ratios.

Ge acts as a fake-isotope of Si, clays will incorporate Ge into their makeup, but Ge is insensitive to other processes (like plant uptake). Ge/Si ratios are useful because they provide insight to what process is controlling the signature of $\delta$³⁰Si in groundwater and stream waters.

When apparent groundwater ages and geochemical tracers are combined together, it provides a much deeper insight into the interactions of groundwater and bedrock, which in turn helps to give a deeper understanding of the time frame of which stream water chemistry has evolved before we have sampled it.

What has been done so far?

To date (September 2024), about 60ish stream samples and 80ish spring samples have been collected! Sampling had be done in June/July of 2023 and also in the summer of 2024.

The samples collected in 2023 have been analyzed for $\delta$ ³⁰Si measurements and Ge/Si ratios. So far, early results suggest that groundwaters in Sagehen Creek basin are close to equilibrium with the bedrock, which is very intriguing!

Future work will include processing the samples collected in 2024 to help paint a better overall picture of the groundwaters in the springs across the basin too.