Linkages of P and Al export at high discharge at the Bear Brook Watershed in Maine

Phosphorus chemistry in streams was evaluated at the paired watershed study at the Bear Brook Watershed, Maine. The West Bear catchment has been treat ed bimonthly since 1989 with 1,800 eq (NH4)(2)SO4 ha(-1) yr(-1). East Bear was the untreated reference watershed. During 1993, concentration of total phosphorus (P) in weekly samples from East and West Bear Brook ranged from 0 to 15 mu g L-1. The median values were 2 and 4 mu g L-1 for East and Wa, Bear, respectively. During a high discharge event in January of 1995, the c oncentration of dissolved P remained relatively constant (less than or equa l to 3 mu g L-1) as pH decreased from 5.63 to 5.08 and from 5.14 to 4.75 in East and West Bear, respectively. The concentration of total P increased t o ca. 60 mu g L-1 during the rising limb of the hydrograph in West Bear, fo ur times the value in East Bear, total P then declined rapidly as discharge remained high followed by an increase. Dissolved Al increased in both stre ams during the episodic acidification. West Bear, the more acidic, had conc entrations of dissolved Al four times those of East Bear (maximum of 1.1 mg L-1 versus 0.25 mg L-1). Acid-soluble particulate Al increased to 0.2 and 4.2 mg L-1 for East and West Bear, respectively, in parallel to total P (bu t was 10(2) greater than total P) and then declined in parallel to total P while discharge remained high. Total P, dissolved P, and particulate Al did not relate to pH. fetal P and particulate Al and Fe were strongly correlat ed. Concurrently, base cations remained relatively constant or decreased sl ightly. Particulate acid-soluble Al exceeded particulate acid-soluble base cations. We hypothesize that the particulate P was occluded in, or adsorbed on, acid-soluble particulate Al(OH)(3). This AI(OH)(3) precipitates as eme rging acidic groundwater degasses CO2 and pH rises. The export of Al and P is greater from the treated watershed because the induced acidification is translocating more Al from soils to the stream. Most of the export of P is related to acid-soluble Al particulate material.