However, d/r ponds do not enjoy an unblemished record of chemical retention and an infinite lifetime. Retention efficiencies vary significantly in time-space and 'events' have been observed in ponds which give rise to short periods of hyper-eutrophication and sporadic export of nutrients and contaminants (e.g., Fig. 2). These 'events' occur at various times of the year and result in downstream 'contamination' that seems to increase with each passing year. Only visible clues (algal blooms) indicate that sampling should have been conducted and the initiation of the 'event' is rarely sampled. We believe that these sporadic d/r pond internal loadings are driven by sediment and/or benthic boundary layer 'events'. A more accurate conceptualization of d/r pond behavior is shown in Fig. 3.

Bacterial oxidation of organic matter in the sediments releases dissolved nutrients to the pore water and degrades colloidal organics which may have sequestered e.g. metals. Respiration and bacterial processes thus create reducing conditions and also release Fe, Mn, dissolved metals, PO₄, and typically NH₃ and other contaminants to the porewater. These processes produce a net flux of contaminants back into the pond water column. This phenomenon will increase over time as the pond sediments are increasingly contaminated during 'aging'. 'Events' occur when the value of λ_R increases (Fig 3) dramatically for short period of time (Fig 2).

Bacterially driven reduction represents a viable and measurable source for an internal loading 'event'. However, the 'event' requires a resuspension of contaminated sediment and porewater over a short time or an accumulation of nutrients and reduced metals for some time period in a BBL and then release. 'Events' can be driven by thermal instabilities in the sediment, runoff, wind, extreme weather, man, animals (watering cattle in rural ponds) and migratory birds. 'Events' have the potential to create significant (multiple years of input) internal loading of nutrients, metals and other contaminants (e.g. pesticide degradation products) to the water column. The sedimentation rate and the sediment resuspension depth determine the impact of the sediment mixing 'event'. The duration of the BBL (days) and the flux from the sediments determines the loading impact of a BBL 'event'. 'Event' release products can be bio-accumulated in plants and animals within the pond resulting in significant ecological degradation. More importantly, 'events' can export nutrients and contaminants (Fig. 2) as governed by the outflow rate and modified by the 're-precipitation rate';C_pλ_s. The frequency and magnitude of 'events' modifies the net loading of the sediment mass (e.g., Fig 3; M_sλ_R=f(t)) and determines the lifetime of the pond. Once the life of a pond is reached, internal loading 'events' and cycling of metals and nutrients creates a greater problem than the problem the pond was originally designed to solve. Small ponds with high loading and infrequent 'events' are likely to reach critical thresholds sooner. Once critical thresholds are passed the pond may become dominated by blue green algae (blue green produce well under nitrogen (NO₃) limitation) and 'events' will be significant. Thus, coupling of pond chemical retention processes, bacterial decay processes and the frequency and mechanisms of 'events' control pond dynamics (not the individual processes).

The broad scope of this project is to establish the fundamentals of the physico-chemo-bio-sediment couples which control the dynamics of d/r ponds and thereby establish their capabilities and lifetime as a function of these dynamics. Size and process-dependent criteria for the design and use of d/r ponds as environmental remediation tools will be significantly enhanced with new understanding of the coupled system dynamics of ponds.