CAN ESTUARIES KEEP PACE WITH PROJECTED RATES OF SEA LEVEL RISE: WHAT HAPPENED TO GALVESTON BAY WHEN SUBSIDENCE WAS 25 TIMES EUSTACY DURING THE 20TH CENTURY?

Estuaries exist because there is a balance between sediment influx, formation of accommodation space and shear stress (erosion). If sediment influx is higher than can be removed by shear stress then the estuary will fill and if formation of accommodation space is at a rate higher than the rate of influx, the estuary will deepen. With the projections of elevated sea level rise over the next century, one pressing question is, will estuaries be able to keep pace with elevated formation of accommodation space? Galveston Bay provides and ideal site to investigate this question. Groundwater withdraw in upper Galveston Bay in the first half of the 20^th Century resulted in a gradient of elevated subsidence rates from the northwestern corner of the bay towards the south and east, with highest rates within the San Jacinto channel of upper Galveston Bay at 9.67 cm/y towards rates of average eustatic sealevel rise of 0.22 cm/y towards the bay center and the southern ends of the bay. Decadal to centennial timescale sedimentation rates were determined with ²¹⁰Pb and ¹³⁷Cs analyses, and found that sedimentation rates in general kept pace with the elevated subsidence rates, with a gradient of sedimentation rates along the subsidence gradient ranging form 5.5 to 0.21 cm/y. However, elevated sedimentation rates also resulted in storage of Hg and likely other particle-bound contaminants with concentrations orders of magnitude higher than background levels, showing that although the sedimentation within Galveston Bay was able to keep pace with subsidence, that the it also resulted in the trapping of elevated levels of particle bound contaminants.