Natural Attenuation of Nitrogen in Groundwater Discharging through a Sandy Beach

Brief description

Changes in horizontal nitrogen (N) concentrations in predischarge groundwater were sampled in a profile of nutrient contaminated groundwater in a shallow sandy beach next to Cockburn Sound, W.A. from November 2004 to August 2005. The degree of mixing and transport in the groundwater, and how this affected groundwater N concentrations, and whether these effects varied with changing seasons were measured.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Sample collection - A 383 m transect of monitoring well and nested beach piezometers were installed perpendicular to the shore and parallel to the groundwater flow at an identified major point source of nutrients to Cockburn Sound (James Point) (See thumbnail). The transect facilitated a two-dimensional study of the predischarge environment, sampling groundwater from 2 or 3 depths beneath the water table at 5 surface locations (labelled A - E) and one surface water sampling location (F). Wells A2, B3 and B4 sampled the confined underlying Tamala Limestone, while all other wells sampled the unconfined superficial Safety Bay Sand Unit. Locations of groundwater well (A - E) and surface-water (F) sampling station. Nest name : Location (eastings) : Well name : Depth bwt (m) A : 383004 : A1 : f10 * : ****** : A2 : f12-20 B : 383014.5 : B1 : 0 * : ******* : B2 : 8 * : ******* : B3 : 12 * : ******* : B4 : 16 C : 383014.5 : C1 : 1 * : ******* : C2 : 3 * : ******* : C3 : 5 D : 383022 : D1 : 1 * : ****** : D2 : 3 * : ****** : D3 : 5 E : 383040 : E1 : 1 * : ****** : E2 : 3 * : ****** : E3 : 5 F : 383058 : SW : 0 bwt = screen depth below the water table f = fully slotted from the water table to bottom of aquifer Groundwater samples were collected at low tide on November 15 2004, April 8 and August 15 2005. These months corresponded to an early summer, late summer and mid-winter seasonal sampling regimes. Wells B3 and B4 were sampled using a peristaltic hammerhead pump and all other wells were sampled using a hand bailer. The bailer was rinsed with at least two times its volume before samples were collected. Water samples for DIN and FRP analysis were collected in 100ml HDPE containers, and samples for C and Si analysis were collected in 100 ml amber glass bottle. All containers were previously acid soaked (10 HCl), rinsed twice with MilliQ Ultrapure water and sir dried. Samples were stored in darkness and on ice and were immediately taken to the laboratory for filtering. Samples were filtered through 0.45 um Sarstedt disposable filters and frozen until DIN analysis (within 10 days) or stored at 4 degrees Celsius for TOC/DOC/TIC analysis (within 2 days). Salinity, temperature, DO and pH were measured in the field immediately upon sample collection using a hand-held TPS WP-81 pH-Cond-Salinity meter with a conductivity probe (part number 122201) and a TPS Aqua-D DO meter with a TPS ED1 sensor. In April a Yeo-Kal 611 multi-parameter water quality meter was used. - Beach sediment characterisation - Soil samples were collected at location C at 1m depth intervals for composition analysis. Grain size distribution was determined by Settling-Tube Analysis (2.22 m, 14 degrees Celsius) and the Gibbs equation (Gibbs et a.. 1971). Carbonate content of sample was determined by acid decomposition, and organic carbon (OC) content was determined by sediment combustion at 540 degrees Celsius for 72 hours.

Notes

Credit
Strategic Research Fund for the Marine Environment (SRFME)

Credit
Carolyn Oldham