According to experimental studies, the efficiency of buffer strips, ponds and wetlands in retaining P varies considerably (Tables 1 and 2).
Table 1. Retention (%) of Total Phosphorus (Tot-P) and Orthophosphate Phosphorus (PO4-P) in Buffer Strip Studies.
45 The interactions of buffer zones and phosphorus runoff
Author Location Source Buffer Retention (%) Comments Area (ha) Width (m) Tot-P PO4-P
Dillaha et al. (1989) Virginia, USA 0.01 4.6 49-85 69-83 Simulated rainfall
9.1 65-93 48-31 Increase of PO4-P
Magette et al. (1987) Maryland, USA 0.01 4.6 41 Less effective over time
9.2 53
Syversen (1995) Norway 0.045 5 45-56 2-77 Natural rainfall,
10 56-85 0-88 slope of 12-17%, and
15 73 10 strips with native grass
Uusi–Kämppä
and Yläranta (1996) Southern Finland 0.063 10 20-36(a) 0-62 Natural rainfall,
increase of PO4-P
Uusi–Kämppä
(unpublished data) Southern Finland 0.063 10 53-78(a) 33-33 During summer 1995
Schwer and
Clausen (1989) Vermont, USA wastewater 26 89 92 Greatest removal in
growing season
Vought et al. (1994) Sweden 8 66 Greatest removal within
first metres
16 95
J. Uusi-Kämppä, E. Turtola, H. Hartikainen and T. Yläranta
Table 2. Retention (%) of Total Phosphorus (Tot-P) and Orthophosphate Phosphorus (PO4- P) in Ponds and Wetlands.
In Norway, Syversen (1994) studied the efficiency of filter strips with widths of 5, 10 and 15 metres planted with native vegetation. More than 50% of the incoming P and 0-88% of DP was removed. In terms of P retention, there were no differences between strips covered with forest and grass. Mander et al. (1991) reported alder forests and willow brushes to be the most effective biotopes for buffer strips. Ten-metre-wide strips were able to adsorb/transform almost 100% of the incoming P.
In buffer strips, the filtration process is found to be of an exponential nature: on the upper part of strips the amount of adsorbed and transformed P is significantly larger than that in the middle and lower part of the buffer (Manderet al., 1991; Voughtet al., 1994). Thus even narrow forest/brush strips may be important in removing nutrients from runoff water.
P retained in buffer strips may be transformed into more mobile forms which may subsequently be lost into an adjacent water body. In an artificially-created rainfall situation, Dillahaet al. (1988) found that buffer strips may increase DP losses. Uusi-Kämppä and Yläranta (1996) obtained similar results from buffer strip plots with native vegetation: the loss of DP was over 50% greater from plots with native vegetation as compared to plots with no buffer strip or with a grass buffer strip. High losses of DP from buffer strip plots with native vegetation may have been due to the release of P from decaying grass residue in the spring. In a model simulation, Leeet al. (1989) found that above-ground biomass in a buffer strip was a more significant source of P than the source soil.
Author Location Source Pond Retention (%) Comments of P Area (ha) Tot-P PO4-P
Sedimentation pond
Brown et al. (1981) Idaho, USA Runoff water 0.09 25-33 Efficiency depended on
flow rate
Hirvonen et al. Southern Finland d.a. of 580 ha 0.18 35 No removal after two
(1996) years
Lindkvist (1992) Southern Sweden d.a. of 650 ha 8 -8 In autumn, increase of
PO4-P
Lindkvist and Southern Sweden d.a. of 650 ha 22 -19 In spring, increase of
Håkansson (1993) PO4-P
Submerged macrophyte pond
Hörberg and Southern Sweden w.w. 1.1 62 Annual P retention 0.03
Kylefors (1991) g/m2d
Mander et al. (1991) Estonia w.w. from a farm 56-67 Greatest removal in
growing season Bioditch
Mander et al. (1991) Estonia w.w. from a farm 60-92 Greatest removal in
growing season Root-zone
Hörberg and Southern Sweden w.w. 0.11 61 Annual P retention
Kylefors (1991) 0.4 g/m2d
Mander et al. (1991) Estonia w.w. from a farm 60-75 Greatest removal in
growing season Artificial Wetland
Braskerud (1994) Southern Norway d.a. of 50-100 ha 0.02-0.09 20-42 Greatest removal in
winter
Jenssen et al. Southern Norway w.w. 0.01 98 A combination of four
(1993, 1995) units
Natural Wetland
Gehrels and Canada Runoff water 18 -22 Increase of PO4-P
Mulamoottil (1990)
Mander et al. (1991) Estonia w.w. 18 27-88 Greatest removal in
summer
Pommel and Switzerland / Runoff water 3 65 65 During stormflows and
Dorioz (1995) France lowflows
d.a. = drainage area w.w. = wastewater
According to several studies, the retention of DP is often rather low (e.g. Dillahaet al., 1988; Leeet al., 1989; Uusi-Kämppä and Yläranta, 1996; Williamset al., 1990). Moreover, a buffer zone may at first act as an efficient sink for P and then turn to a source of P (Richardson, 1985; Dillaha et al., 1988, 1989; Magetteet al., 1989; Vanek, 1991). This may happen when environmental conditions change, when the soil material becomes gradually enriched with P or when the assimilative capacity of a buffer zone is exhausted, allowing previously trapped P to be released from the filter vegetation and soil as DP. Dillaha et al. (1988) assumed that DP removal should decrease with time as filtration decreases, the adsorption capacity of vegetation is saturated and surface soil P sorption sites become occupied. Both natural and artificial wetlands have been used for removing P from domestic wastewater and agricultural runoff (e.g. Richardson, 1985; Mander et al., 1991; Baker, 1992; Mitch, 1992; Braskerud, 1994; Jenssen et al., 1995). Artificial wetland ecosystems with aquatic vascular plants, vegetated bioponds and bioditches on natural soil, root zone systems in water bodies, and vegetated riparian buffer strips were able to adsorb and/or transform P (Mander et al., 1991). In Norway, an artificial wetland system planted with Phragmites and Typha and modified with a fabricated porous medium (Leca 0-4 mm) with high P adsorption capacity showed an average removal of 97% for P over a period of 18 months (Jenssenet al., 1993, 1994).
Wetlands may convert some PP to plant available PO4-P, thus contributing to downstream
eutrophication problems (e.g. Gehrels and Mulamoottil, 1990). This conversion is considered to be due to the intense leaching of decaying vegetation and high water levels, which induces anaerobic conditions, thus increasing the solubility of phosphate.
In Finland, Hirvonen (1994) reported on an experimental pond, built in a ditch to retain P from agricultural field waters. The retention of P was best in the first year, when as much as two thirds of the incoming P was trapped. In the second year only one fifth of the P was retained, and over the last two years the pond did not retain any P (Hirvonen et al., 1996). Richardson (1985) reported that wetlands used for wastewater filtration became P-saturated in just a few years.
Wetlands may temporarily act as a sink or a source for P. In summer, the wetlands usually retain P because of sedimentation. In autumn, however, the high flows resuspend the sediment and carry it forward in the watercourse. Therefore, P retained during the summer period can be a major source of nutrient transport in the autumn (Svendsen, 1992; Taponen, 1995). Moreover, the water-sediment interface may become anoxic also during summer, increasing the release of PO4-P.