The objectives of this project were to develop a basic understanding of the release rate and subsequent fate and transport of munitions constituents (MCs) in water and in sediment. The ability to characterize, assess, and predict potential MC source loading and distribution has significant implications for Department of Defense (DoD) and Department of Navy range sustainability initiatives. DoD will gain critical information for making scientifically defensible risk management decisions about underwater ordnance leave-in-place (LIP) mitigation and blow-in-place (BIP) versus removal options. In addition to explosive blast (safety) considerations, future regulatory emphasis will likely require an assessment of potential underwater ordnance contamination and mitigation efforts that could include water and sediment quality issues.
As a baseline scenario, this project addressed the amount of MC introduced into the environment from the case of a single breached munition. The information determined for MC release from a single breached shell was incorporated into the calibrated hydrodynamic/transport model to predict fate and transport of the MC and degradation products. To address the specific release function and its predictive ability, analytical, empirical, and numerical modeling studies of MC source release from breached shells under various hydrodynamic and shell integrity conditions were conducted. Release rates were developed and validated for a single breached shell in three nominal release scenarios: a shell on the surface of bottom sediment, a shell from a low-order detonation above the bottom in the water column, and a shell buried in sediment. The semi-analytical release rate function was validated by results from both the empirical study and the numerical modeling study, using the FLUENT model. The calibrated model, TRIM2D, and Environmental Protection Agency WASP7 model were both used to simulate the fate and transport of trinitrotoluene explosive; 2-methyl-1,3,5-trinitrobenzene (TNT) released by design from a breach shell in San Diego Bay, California, and Elizabeth River, Virginia.
The release function developed semi-analytically assumes that the release through a breach in a munition casing can be determined by the following five key parameters: (1) the growth of the breach hole (as the radius of the hole, (b)); (2) the radius of the cavity formed due to loss of mass released from inside the shell, (R)); (3) the dissolution rate (µ) from solid to aqueous phases of the MC inside the shell casing; (4) the outside ambient current (U) to which the casing hole is exposed; and (5) the hydrodynamic dispersion coefficient (D). This function dictates the release rate be characteristically governed by two Reynolds Numbers: the current-based Reynolds Number (Ub/D) and the dissolution-based Reynolds Number (µR/D). Release rate is governed by current (U) and dissolution rate (µ) for the scenarios (µR/D >>n Ub/D) and (µR /D << Ub/D), respectively. Empirical studies were conducted for release scenarios both in the water column and in sediment. For a current-controlled scenario, release rates are proportional to the ambient current speed, and therefore, release rates in the water column are much greater than in sediment. However, total time for a complete MC release from a test shell (L = 24”, r = 8”, 8-kg MC weight) also depends on the breach hole size (e.g., crack area) and the dissolution rate, which can result in the total release time varying from a few days to several hundred years.
The fate and transport modeling study shows that predicted TNT concentrations are on the order of nanograms per liter (ng/L) in the vicinity of the release locations for a single test shell. This provides the basis for an order of magnitude estimate for worst-case scenarios. For example, to generate TNT concentrations to the micrograms per liter (µg/L) level in the water column, it would take 1,000 shells of the same shell integrity to be co-located at a single site.
A basic understanding of processes and governing factors for the release rate and the fate and transport of MC in the marine environment was developed. Predictive modeling capabilities were also developed for these processes. With this data, DoD will be better equipped to make technically defensible managerial decisions for sites with underwater ordnance.