Mobile colloids in soils and their underlying strata may play an impor
tant role in the translocation of some contaminants from surface sourc
es to groundwater. This study was conducted to evaluate the role of ad
sorbed natural organic matter (NOM) in the transport of submicron soil
colloids through a commonly occurring type of saprolite in North Caro
lina. Intact saprolite columns from 4 m below the soil surface were us
ed to study the movement of a conservative tracer ((H2O)-H-3) and of s
oil colloids with and without adsorbed NOM. For natural (i.e., untreat
ed) soil colloids having high colloidal stability due to adsorbed NOM,
the filter efficiency of the saprolite decreased rapidly to zero as i
ncreasing amounts of colloids were deposited on the pore walls in the
saprolite (blocking effect). Colloid breakthrough curves exhibited lit
tle tailing, indicating that colloid deposition was largely irreversib
le. The colloids were excluded from about 33% of the water-filled pore
space, resulting in faster transport of colloids as compared to (H2O)
-H-3. When the colloids were treated with NaOCl to remove adsorbed NOM
, colloidal stability and mobility were strongly decreased. For these
suspensions the filter efficiency of the columns increased as increasi
ng amounts of colloids were deposited in the saprolite (filter ripenin
g). After addition of small amounts of humic acid (1 mg L(-1)) to the
NaOCl-treated colloids, they exhibited very similar transport behavior
as the untreated soil colloids. Stabilization of colloids by NOM and
the possible occurrence of the blocking effect or filter ripening must
be considered in future models of subsurface colloid transport.