Document Type

Technical Report

Publication Date

12-1-2014

Keywords

Python programming, water monitoring, Arkansas, PECCI

Abstract

In karst settings, hydrograph separations using isotopic tracers are commonly and effectively used to quantify the proportions of rain rapidly delivered to springs along fractures and conduits during storm events. Dissolved inorganic carbon (DIC) is an effective, non‐conservative tracer for use in hydrograph separations of karst waters because of the ubiquitous nature of carbon in the sources of waters to caves and springs and unique concentrations and isotopic compositions of carbon inputs. DIC concentration and isotopic composition (δ¹³C‐DIC) in rain are typically calculated based on atmospheric carbon dioxide (CO₂) using equilibrium carbonate reactions and stable carbon isotope fractionation values. As atmospheric CO₂ changes, traditional assumptions applied in attaining calculated values can result in error, and better estimates of rain DIC are needed. The concentration and isotopic composition of rain DIC in the karst of northwestern Arkansas was calculated using Python™ programming language based on local atmospheric CO₂ and rain pH data from 2011 to 2013. Python™ provides an open‐source code and rapid means to complete iterative calculations, and the PECCI code (Python™ Estimation for Carbon Concentration and Isotopes) can be used for rain DIC calculations in other areas. Measured northwestern Arkansas atmospheric CO₂ had a median concentration of 397.7 ± 4.3 ppm and increased slightly over three years and median δ¹³C‐CO₂ was ‐8.5 ±0.4 ‰. Rain samples exhibited a median pH of 5.6 ±0.4. Calculated rain DIC ranged from 0.17 to 0.34 mg/L and δ13C‐DIC ranged from ‐8.5‰ to ‐8.2‰ between 5 and 30 °C. At an average annual temperature of 14.6 °C, rain DIC was calculated to be 0.25 mg/L and δ¹³C‐DIC was ‐8.34 ‰. Although the variations in DIC are small, the concentration and isotopic composition of end‐member sources in hydrograph separations controls the final hydrologic budget calculations. The PECCI code can be modified to calculate rain DIC for otherstudy sites or time periods.

Report Number

MSC370

Page

26

COinS