We present an empirical study of contamination in deep , rapid , and wide-field optical follow-up searches of gravitational wave sources from Advanced LIGO/Virgo ( ALV ) .
We utilize dedicated observations during four nights of imaging with the Dark Energy Camera ( DECam ) wide-field imager on the Blanco 4-m telescope at CTIO .
Our search covered \sim 56 deg ^ { 2 } , with two visits per night separated by \approx 3 hours , in i - and z -band , followed by an additional set of griz images three weeks later to serve as reference images for subtraction , and for the purpose of identifying galaxy and stellar counterparts for any transient sources .
We achieve 5 \sigma point-source limiting magnitudes of i \approx 23.5 and z \approx 22.4 mag in the coadded single-epoch images .
We conduct a search for transient objects that can mimic the i - z color behavior of both red ( i - z > 0.5 mag ) and blue ( i - z < 0 mag ) kilonova emission , finding 11 and 10 contaminants , respectively .
Independent of color , we identify 48 transients of interest .
Additionally , we leverage the rapid cadence of our observations to search for sources with characteristic timescales of \approx 1 day and \approx 3 hours , finding no potential contaminants .
We assess the efficiency of our pipeline and search methodology with injected point sources , finding that we are 90 % ( 60 % ) efficient when searching for red ( blue ) kilonova-like sources to a limiting magnitude of i \lesssim 22.5 mag .
Applying these efficiencies , we derive sky rates for kilonova contaminants in the red and blue regimes of \mathcal { R } _ { red } \approx 0.16 deg ^ { -2 } and \mathcal { R } _ { blue } \approx 0.80 deg ^ { -2 } .
The total contamination rate , independent of color , is \mathcal { R } _ { all } \approx 1.79 deg ^ { -2 } .
We compare our derived results to optical follow-up searches of the gravitational wave events GW150914 and GW151226 and comment on the outlook for GW follow-up searches as additional GW detectors ( e.g. , KAGRA , LIGO India ) come online in the next decade .