Going through my pix I found this - this duck was hiding in reeds near the male gadwall, then after half an hour came out. Long wait :) Been flicking through the RSPB mallard identifier and gadwall identifier pages til I've gone cross-eyed. Neither a white patch on the wing nor a blue patch to help me. I'm minded towards gadwall as it was close to the male and the speckling around the throat seems finer.
http://www.flickr.com/photos/steve_dewey/sets/72157623533703702/ (there's birds there - and some are actually sharp!)
I'd say yes to a Gadwall,
she has an orange beak, Mallard have a yellow one.(i think)
Hopefully this may help, Male and Female Gadwall together
In your image and this one the distinctive feature is the Two-Tone Bill [Dark top and orange sides :)
| My Images | Newport Wetlands on Flickr @Barman58
Thanks for additional help :)