Reports
Hart-Miller Island well monitoring: anaerobic sampling study - groundwater quality assessment
2005, Hill, J.M.
File Reports, Coastal and Estuarine Geology, File Report 2005-05
Executive Summary
The geochemistry of the groundwater that flows through the dike at the Hart-Miller Island Dredged Material Containment Facility was completed in order to:
- Determine the major geochemical processes controlling the groundwater chemistry to assess how the facility operations affect the water quality
- Assess the level of concern of the measured parameters as to their potential environmental impact;
- Evaluate the chemical state of the groundwater in order to determine the most effective sampling protocol, and;
- Determine the quality/usability of the data obtained during previous and contemporary monitoring events;
Sample collection lasted a year and a half (June 2002 - Nov. 2003), a period that spanned a low to high rainfall period. Samples were collected on an approximately monthly basis based on logistics and weather conditions. Six wells were sampled, two in the North Cell and four in the South Cell. Two of the wells, one at each cell, were purged with oxygen free nitrogen and sampled under strict anaerobic conditions; the other four wells were not. Of the four aerobic wells, two of the wells were monitoring wells used prior to this study; measurements from these wells were to be used for evaluation and comparison with new wells. The last two wells were newly installed wells to compare and reference the old and anoxic wells.
Groundwater in the containment facility responds to the source of water input to the system and how the material is handled. Three conditions, with different responses were noted in this report:
- Sediments are continually inundated with the input of fresh Bay water. Maintaining flooded conditions produces groundwater that is similar to the porewaters found in the sediments when they were in the Bay. This was the case in the North Cell.;
- Sediments are exposed to the atmosphere during dewatering operations, and the only source of water input is freshwater from rainfall. If the oxidation rate is slower than the flushing rate, then the oxidation by-products will be associated with the freshwater and inversely related to the saltwater species associated with the buried sediments. This was the case in the South Cell during the study period.
- This case is similar to 2., in that the sediments are exposed to the atmosphere, however due to site conditions and operations, the oxidation rate is faster than the flushing rate. This condition results in the oxidation products being associated with the native salt content of the sediment. This was not seen at HMI, but was the case at Pearce Creek.
Oxidation of sulfide minerals in the sediment produces acid; this would be expected to lower the pH significantly. This would occur if the buffer capacity of the system becomes overwhelmed allowing acidic conditions develop. More acidic conditions would overtax the cation exchange capacity of the sediments (releasing adsorbed metals), oxyhydroxides would not form and the dissolved phase of the metals would dominate. Consequently, much higher metal and nutrient levels would be expected. Oxidation is evident in the South Cell wells, however currently the buffer capacity of the sediment and porous media of the dike is sufficient to maintain near neutral pH levels. Under these conditions, free acid formation is inhibited and the associated acid leachate formation and transport through the dike is not significant.
In regard to sampling the groundwaters at HMI and other upland disposal sites, care must be taken to properly collect samples. Due to the chemistry of the groundwater. All of the samples collected in this study showed reduced oxidation levels to a varying extent. Consequently the interaction of the sample with oxygen at the time of collection can radically alter the parameters measured; specifically redox species and minor components like nutrients, and organics that can be lost from solution through precipitation and absorption. Based on the data, the wells do not need to be maintained under anoxic conditions in future studies. However, the samples must be collected by low flow sampling methods: low flow collection rates are the volume per unit time equal to or less than the flow through the well (based on linear velocity and cross-sectional area). If the wells are exposed to the atmosphere, it will take longer to achieve stable electrochemical readings at the site; but once stable, these measurements are comparable to the readings taken at wells purged with nitrogen. Groundwater collected for dissolved chemical species should use an in-line filter capsule to avoid contact with atmospheric oxygen, and to preclude the loss of precipitates formed after collection of the samples. Once collected, the groundwaters need to be preserved as soon as possible, especially in regard to the acidification of the samples for trace metal analyses. Acidification of the sample will dissolve any newly formed colloidal metal precipitates, and prevent settling due to aging and agglomeration.
Samples collected prior to 1999 did not follow this protocol, and used high flow techniques that thoroughly oxygenated the samples prior to collection. Data from these samples that would be considered reliable are the major dissolved seawater species, and possibly ammonium; these species are relatively unaffected by precipitation, absorption and rapid oxidation. Ammonium is problematic; depending on the state of the system, ammonium could be swept from the system as ammonia. The URS samples, collected contemporaneously with this study, used the same low flow protocol. However, the samples were filtered after collection and transported to an out-of -state lab. The samples were not filtered in-line, so there was a long period where the samples were exposed to the atmosphere, and the species that precipitated from solution as a result of this exposure were lost and not accounted for in flux measurements. The “totals” analyses can be considered upper limits of the dissolved load, since there could be a substantial contribution to this measure due to solids introduced from the well site.
Future monitoring of the groundwater is advisable to determine the flux of material to the Bay from the groundwater. Currently there appears to be no problem, the groundwater concentrations of species of interest are low. However, the groundwater chemistry reflects variations in the climatic conditions, sediment composition, and changes in use and operations at HMI. As HMI evolves from one stage of use to the next, it is important to monitor and document the input to the Bay from groundwater to assure that no problem develops. To this end, the newly installed wells should be used in preference to the old monitoring wells. The old wells show anomalous behavior either due to their age or the vagaries of their construction.
