Research Objective

The major goal of this project is to develop a method to analyze trace metals in travertine samples. So that an understanding of the amount of municipal supply water in the spring water can be calculated. And thus, some of the effects urbanization has on groundwater can be better understood. Trace metal ratios such as Mg/Ca, Sr/Ca, and Ba/Ca help constrain the amount of municipal water in a given sample along with the ⁸⁷Sr/⁸⁶Sr isotopes values not measured in this study due to limitations of this instrument (Christian et al., 2012; Beal et al., 2020).  Using travertines as a geologic proxy for the recent temporal record of water would allow us to quantify the rate at which urbanization effects have modified water quality over time in the Bull Creek Watershed.  As the travertine layers get older, it is expected that the trace metals ratios should be more similar to that of modified municipal supply water rather than that of the modified rural water of Austin because of the small amount of urbanization occurring at the time.

Justification

The city of Austin and its underlying watersheds are currently going through rapid urbanization. Austin’s natural terrain is highly karstic and the Edwards aquifer underlying Austin is used as drinking water for San Antonio and other areas in central Texas (DeMott, 2007). This ongoing process of urbanization should be better understood so that as Austin continues to expand or other cities’ populations continue to increase in a similar geologic setting where the supply water is geochemically different than the local water, water resources are protected from failing water infrastructure. Bull Creek watershed poses a valuable case study as urbanization within the watershed has largely occurred within the past 45 years when the associated water infrastructure began developing in the same time frame. The temporal record of water in Loop 360/2222 travertine could indicate the rate of water infrastructure failure over time. The trace metal ratios present in the travertine could also constrain the ⁸⁷Sr/⁸⁶Sr values which have a relationship to the municipal supply water and cretaceous limestone water of Austin (Christian et al., 2012; Beal et al., 2020). This information would pose beneficial to future urban planning and the development of water infrastructure.

The trace elemental ratios of Mg/Ca, Sr/Ca, Ba/Ca and Na/Ca were selected they can be easily incorporated into calcite and be useful for understanding the waters the calcite precipitated from. These ratios and their effective distribution coefficients can help determine information about the source of the water recharge, groundwater flow paths, fluid mixing, and water-rock interaction (Banner, 1995). The method developed to analyze the travertine for these trace element ratios should be sensitive enough to detect ppm change in each of the analytes.

Review of Relevant Work

In DeMott (2007) a sample travertine was extracted in 2006 from a road cut at Loop 360/2222 located within the Bull Creek watershed.  The sample has approximately 28 layers and has been estimated to have begun forming after the construction of the road in 1976 where the road intersected a perched water table and began precipitating travertine.  Along with the travertine sample, water samples of the spring and calcium substrate experiments have been collected intermittently since 1997. Soil and rock samples were collected in 2006 from the spring location. The water and calcite substrate and travertine samples were analyzed through the ICP-MS on solution mode Ca, K, Mg, Na, Sr, Ag, Al, As, B, Ba, Co, Cu, Fe, Li, Mn, Mo, Pb, Rb, Se, Si, U, and V. For preservation purposes the travertine has been epoxied and stored and thick sections of the travertine were made and will be prepared for the upcoming analysis.

Jocham et al. (2012) discuss the analytical problems associated with the LA-ICP-MS trace metal analysis for calcium carbonates. They highlight key interferences and suggest using 193 nm lasers for more accurate and precise analysis of chalcophile and siderophile trace elements to limit the amount of elemental fractionation and mass-load dependent matrix effects on those. As well as compare the NIST SRM 610 and 612 and MACS-3 as standards for carbonates. Thompson et al., (2021) describe the effects of spot sizes, laser fluences, and calibration methods they find that NIST SRM 612 and USGS BCR-2G as secondary reference material allow for the best accuracy. Sürmelihindi et al. (2013) explain their methods for analyzing laminated carbonate deposits using the LA-ICP-MS and method by which they scanned their sample and how porosity and the difference of grain size in the layers of the sample factored into their method.

Materials Methods

For the development of this method using the LA-ICP-MS and analysis of trace metals, I will prepare my sample by making sure the surface of the travertine is flat and can fit within the laser ablation cell. A correction for the epoxy, if necessary, will be made by ablating just the epoxy to see what trace elements could be present and could be contributing to higher than expected values for the trace elements in the travertine. The quality controls will be the epoxy blank and the procedural blank and at least 1 replicate will be taken for every 10 samples ablations. The external standards that will be used for the procedure should be NIST SMR 610, 612 and USGS MACS-3 for both non-matrix matched and matrix matched standards. Ca43 will be used as an internal standard for the analysis. Calibration will also occur every 10 samples taken and at least 1 replicate per calibration period should be observed to account for the precision of the method. To avoid high levels of interference He will be the carrier gas. For every layer of the travertine at least 2 samples will be taken to account for seasonality. Due to the variation of grain size and pore space, the ablation should be around a 100 nm spot using the 193 nm laser. The laser line scan should later account for areas of high porosity and non-relevant mineral phases. Because of the variations in the thickness of each travertine layer, the number of spots ablated will be variable, with a distance of 14.6 μm between each spot.

Possible Outcomes

The calcite present in travertine can be used as an environmental proxy for the recent temporal record of the water in the Bull Creek watershed. The ⁸⁷Sr/ ⁸⁶Sr isotopic ratio coupled with the trace elemental ratio should allow us to examine how failing infrastructure effects and the relative abundance of municipal supply water in the precipitating travertine spring change over time. If, as the travertine layers get younger (more recent), the trace elemental ratio becomes more indicative of municipal supply water, then water infrastructure in the area is most likely contributing to spring flow. The data collected from this analysis will help inform future work about the urbanization of watersheds and constrain the conceptual model on the evolution of groundwater flow in the greater Austin area.

Timeline and Budget

Since there are 28 layers in the travertine, equating to roughly 36 cm of line scan. Included, should be calibration standards and a replicate line scan. Since the cost of the LA-ICP-MS is $73/ hr, with a higher-end estimation of 4 hours of laser time (including sample run and cool down time) the proposed budget is $292.

Table 1: Proposed Timeline