Sorghum 



y 



Sand 



Water 



Figure 1 . The system of Metcalf and coworkers^ mimicking a "farm pond" is used to 

 estimate bioaccumulation potential and biodegradation. Tank dimensions 

 are 35x25 x 1 5 cm., with approximately 4 I. of water and 1 kg of sand. The 

 water is innoculated with pond scum, sorghum (Hordendum vulgaris) is 

 planted in the sand, and catepillars (Estigme sp.), Daphnia spp., snail 

 (Physa sp.), and mosquitof ish (Gambusia sp.) are added on a fixed schedule 

 following treatment with radiolabeled chemical. 



Although this system or its successors provide the only quantitative view of the 

 environmental fate of chemicals in a holistic system, ecologists and many chemists 

 have been reluctant to support the approach. The experiments are run without 

 controls or replicates, and introduction of chemicals is artificial, as are the biological 

 structure and trophic/energy relationships. There are practically no criteria for what 

 could be termed a correct experiment. It has been argued that even the chemical fate 

 in such experiments is out of scale in both time and space; partitioning between 

 various compartments of the system may not be realistic. 



Correct or not, the "Metcalf system" has sparked a variety of approaches similar in 

 concept if not structure. "Model feedlots" with mice fed drugs were suspended over 

 the aquatic system. "Rice paddies" were constructed to look at return flows from 

 irrigation and runoff. Such flows have also been examined in "soil-plant" systems. 

 Aquatic systems were improved in design to permit separation of predators (fish, 

 crayfish) from prey (Daphnia). Some of the chemicals studied in these systems are 

 shown in Table 2. 



By the mid-1970s, about a dozen laboratories were developing such systems. 

 Metcalfe/ al. focused on the terrestrial part of their system by using a "terrestrial 

 monoculture" approach^ which they then coupled to their aquatic system for a 

 "physical laboratory model ecosystem" (Figure 2). In parallel, the "Terrestrial 

 Microcosm Chamber" (TMC)*'^ was developed in Corvallis and the "micro- 

 agroecosystem"^ (Figure 3) at Beltsville, Maryland. These involve at least an order of 

 magnitude greater scale of the soil system and have somewhat different bases. The 

 TMC system and microagroecosystem are both designed for chemical mass balance 



154 



