Document Type

Thesis

Abstract

In urban environments, runoff from impervious surfaces is a widespread problem that exacerbates channel erosion and degrades water quality. Specifically, overland flow carries more sediment, debris, and anthropogenic byproducts into urban streams making it increasingly important to determine the volume, strength, and carrying capacity of this surface runoff component in storm hydrographs. In order to better quantify what percentage of rainwater becomes runoff per individual storm event, cosmogenic Sodium- 24 (24Na, T1/2 = 16 hours) was measured in rainfall and immediate surface runoff in the Williamsburg, Virginia area. 24Na is a short-lived isotope with a half-life of 16 hours, creating a time sensitive sample. However, it is ideal for tracking rainfall from a single event, as there will be no resident 24Na in the system to account for, unlike previous methods of quantifying percent runoff such as Oxygen-18, electrical conductivity, or dissolved silica. Rainfall was collected via a rain barrel fed by a gutter system trapping precipitation that falls on a nine square meter roof. Storm surface runoff was collected using ISCO water samplers and the manual filling of carboys at peak outflow of chosen surface discharge points. Focus was in an urbanized catchment as the second location of an underdeveloped watershed proved challenging to sample. Samples were concentrated for sodium using ion exchange resin and the resin was gamma counted for 24Na. We successfully measured 24Na in rainwater samples, with values ranging from 392 — 591 atoms/L with a mean of 493 atoms/L, which are consistent with Komura et al. (2006) values of 30-1500 atoms/L with a mean of 520 atoms/L. Values for 24Na in runoff are identical to those in rain samples, showing a complete recovery of 24Na from rainfall to runoff, unlike other isotopes such as the measured 7Be, making a 24Na a good hydrologic tracer.

Date Awarded

2016

Department

Geology

Advisor 1

James M. Kaste

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